erl - The Erlang emulator.
Please see following description for synopsis
erl(1) User Commands erl(1)
NAME
erl - The Erlang emulator.
DESCRIPTION
The erl program starts an Erlang runtime system. The exact details (for
example, whether erl is a script or a program and which other programs
it calls) are system-dependent.
Windows users probably want to use the werl program instead, which runs
in its own window with scrollbars and supports command-line editing.
The erl program on Windows provides no line editing in its shell, and
on Windows 95 there is no way to scroll back to text that has scrolled
off the screen. The erl program must be used, however, in pipelines or
if you want to redirect standard input or output.
Note:
As from ERTS 5.9 (Erlang/OTP R15B) the runtime system does by default
not bind schedulers to logical processors. For more information, see
system flag +sbt.
EXPORTS
erl <arguments>
Starts an Erlang runtime system.
The arguments can be divided into emulator flags, flags, and
plain arguments:
* Any argument starting with character + is interpreted as an
emulator flag.
As indicated by the name, emulator flags control the behav-
ior of the emulator.
* Any argument starting with character - (hyphen) is inter-
preted as a flag, which is to be passed to the Erlang part
of the runtime system, more specifically to the init system
process, see init(3).
The init process itself interprets some of these flags, the
init flags. It also stores any remaining flags, the user
flags. The latter can be retrieved by calling init:get_argu-
ment/1.
A small number of "-" flags exist, which now actually are
emulator flags, see the description below.
* Plain arguments are not interpreted in any way. They are
also stored by the init process and can be retrieved by
calling init:get_plain_arguments/0. Plain arguments can
occur before the first flag, or after a -- flag. Also, the
-extra flag causes everything that follows to become plain
arguments.
Examples:
% erl +W w -sname arnie +R 9 -s my_init -extra +bertie
(arnie@host)1> init:get_argument(sname).
{ok,[["arnie"]]}
(arnie@host)2> init:get_plain_arguments().
["+bertie"]
Here +W w and +R 9 are emulator flags. -s my_init is an init
flag, interpreted by init. -sname arnie is a user flag, stored
by init. It is read by Kernel and causes the Erlang runtime sys-
tem to become distributed. Finally, everything after -extra
(that is, +bertie) is considered as plain arguments.
% erl -myflag 1
1> init:get_argument(myflag).
{ok,[["1"]]}
2> init:get_plain_arguments().
[]
Here the user flag -myflag 1 is passed to and stored by the init
process. It is a user-defined flag, presumably used by some
user-defined application.
FLAGS
In the following list, init flags are marked "(init flag)". Unless oth-
erwise specified, all other flags are user flags, for which the values
can be retrieved by calling init:get_argument/1. Notice that the list
of user flags is not exhaustive, there can be more application-specific
flags that instead are described in the corresponding application docu-
mentation.
-- (init flag):
Everything following -- up to the next flag (-flag or +flag) is
considered plain arguments and can be retrieved using
init:get_plain_arguments/0.
-Application Par Val:
Sets the application configuration parameter Par to the value Val
for the application Application; see app(4) and application(3).
-args_file FileName:
Command-line arguments are read from the file FileName. The argu-
ments read from the file replace flag '-args_file FileName' on the
resulting command line.
The file FileName is to be a plain text file and can contain com-
ments and command-line arguments. A comment begins with a # charac-
ter and continues until the next end of line character. Backslash
(\\) is used as quoting character. All command-line arguments
accepted by erl are allowed, also flag -args_file FileName. Be
careful not to cause circular dependencies between files containing
flag -args_file, though.
The flag -extra is treated in special way. Its scope ends at the
end of the file. Arguments following an -extra flag are moved on
the command line into the -extra section, that is, the end of the
command line following after an -extra flag.
-async_shell_start:
The initial Erlang shell does not read user input until the system
boot procedure has been completed (Erlang/OTP 5.4 and later). This
flag disables the start synchronization feature and lets the shell
start in parallel with the rest of the system.
-boot File:
Specifies the name of the boot file, File.boot, which is used to
start the system; see init(3). Unless File contains an absolute
path, the system searches for File.boot in the current and
$ROOT/bin directories.
Defaults to $ROOT/bin/start.boot.
-boot_var Var Dir:
If the boot script contains a path variable Var other than $ROOT,
this variable is expanded to Dir. Used when applications are
installed in another directory than $ROOT/lib; see sys-
tools:make_script/1,2 in SASL.
-code_path_cache:
Enables the code path cache of the code server; see code(3).
-compile Mod1 Mod2 ...:
Compiles the specified modules and then terminates (with non-zero
exit code if the compilation of some file did not succeed). Implies
-noinput.
Not recommended; use erlc instead.
-config Config:
Specifies the name of a configuration file, Config.config, which is
used to configure applications; see app(4) and application(3).
-connect_all false:
If this flag is present, global does not maintain a fully connected
network of distributed Erlang nodes, and then global name registra-
tion cannot be used; see global(3).
-cookie Cookie:
Obsolete flag without any effect and common misspelling for -set-
cookie. Use -setcookie instead.
-detached:
Starts the Erlang runtime system detached from the system console.
Useful for running daemons and backgrounds processes. Implies
-noinput.
-emu_args:
Useful for debugging. Prints the arguments sent to the emulator.
-env Variable Value:
Sets the host OS environment variable Variable to the value Value
for the Erlang runtime system. Example:
% erl -env DISPLAY gin:0
In this example, an Erlang runtime system is started with environ-
ment variable DISPLAY set to gin:0.
-epmd_module Module (init flag):
Configures the module responsible to communicate to epmd. Defaults
to erl_epmd.
-eval Expr (init flag):
Makes init evaluate the expression Expr; see init(3).
-extra (init flag):
Everything following -extra is considered plain arguments and can
be retrieved using init:get_plain_arguments/0.
-heart:
Starts heartbeat monitoring of the Erlang runtime system; see
heart(3).
-hidden:
Starts the Erlang runtime system as a hidden node, if it is run as
a distributed node. Hidden nodes always establish hidden connec-
tions to all other nodes except for nodes in the same global group.
Hidden connections are not published on any of the connected nodes,
that is, none of the connected nodes are part of the result from
nodes/0 on the other node. See also hidden global groups;
global_group(3).
-hosts Hosts:
Specifies the IP addresses for the hosts on which Erlang boot
servers are running, see erl_boot_server(3). This flag is mandatory
if flag -loader inet is present.
The IP addresses must be specified in the standard form (four deci-
mal numbers separated by periods, for example, "150.236.20.74".
Hosts names are not acceptable, but a broadcast address (preferably
limited to the local network) is.
-id Id:
Specifies the identity of the Erlang runtime system. If it is run
as a distributed node, Id must be identical to the name supplied
together with flag -sname or -name.
-init_debug:
Makes init write some debug information while interpreting the boot
script.
-instr (emulator flag):
Selects an instrumented Erlang runtime system (virtual machine) to
run, instead of the ordinary one. When running an instrumented run-
time system, some resource usage data can be obtained and analyzed
using the instrument module. Functionally, it behaves exactly like
an ordinary Erlang runtime system.
-loader Loader:
Specifies the method used by erl_prim_loader to load Erlang modules
into the system; see erl_prim_loader(3). Two Loader methods are
supported:
* efile, which means use the local file system, this is the
default.
* inet, which means use a boot server on another machine. The flags
-id, -hosts and -setcookie must also be specified.
If Loader is something else, the user-supplied Loader port program
is started.
-make:
Makes the Erlang runtime system invoke make:all() in the current
working directory and then terminate; see make(3). Implies -noin-
put.
-man Module:
Displays the manual page for the Erlang module Module. Only sup-
ported on Unix.
-mode interactive | embedded:
Indicates if the system is to load code dynamically (interactive),
or if all code is to be loaded during system initialization (embed-
ded); see code(3). Defaults to interactive.
-name Name:
Makes the Erlang runtime system into a distributed node. This flag
invokes all network servers necessary for a node to become distrib-
uted; see net_kernel(3). It is also ensured that epmd runs on the
current host before Erlang is started; see epmd(1).and the
-start_epmd option.
The node name will be Name@Host, where Host is the fully qualified
host name of the current host. For short names, use flag -sname
instead.
-noinput:
Ensures that the Erlang runtime system never tries to read any
input. Implies -noshell.
-noshell:
Starts an Erlang runtime system with no shell. This flag makes it
possible to have the Erlang runtime system as a component in a
series of Unix pipes.
-nostick:
Disables the sticky directory facility of the Erlang code server;
see code(3).
-oldshell:
Invokes the old Erlang shell from Erlang/OTP 3.3. The old shell can
still be used.
-pa Dir1 Dir2 ...:
Adds the specified directories to the beginning of the code path,
similar to code:add_pathsa/1. Note that the order of the given
directories will be reversed in the resulting path.
As an alternative to -pa, if several directories are to be
prepended to the code path and the directories have a common parent
directory, that parent directory can be specified in environment
variable ERL_LIBS; see code(3).
-pz Dir1 Dir2 ...:
Adds the specified directories to the end of the code path, similar
to code:add_pathsz/1; see code(3).
-path Dir1 Dir2 ...:
Replaces the path specified in the boot script; see script(4).
-proto_dist Proto:
Specifies a protocol for Erlang distribution:
inet_tcp:
TCP over IPv4 (the default)
inet_tls:
Distribution over TLS/SSL
inet6_tcp:
TCP over IPv6
For example, to start up IPv6 distributed nodes:
% erl -name test@ipv6node.example.com -proto_dist inet6_tcp
-remsh Node:
Starts Erlang with a remote shell connected to Node.
-rsh Program:
Specifies an alternative to rsh for starting a slave node on a
remote host; see slave(3).
-run Mod [Func [Arg1, Arg2, ...]] (init flag):
Makes init call the specified function. Func defaults to start. If
no arguments are provided, the function is assumed to be of arity
0. Otherwise it is assumed to be of arity 1, taking the list
[Arg1,Arg2,...] as argument. All arguments are passed as strings.
See init(3).
-s Mod [Func [Arg1, Arg2, ...]] (init flag):
Makes init call the specified function. Func defaults to start. If
no arguments are provided, the function is assumed to be of arity
0. Otherwise it is assumed to be of arity 1, taking the list
[Arg1,Arg2,...] as argument. All arguments are passed as atoms. See
init(3).
-setcookie Cookie:
Sets the magic cookie of the node to Cookie; see
erlang:set_cookie/2.
-shutdown_time Time:
Specifies how long time (in milliseconds) the init process is
allowed to spend shutting down the system. If Time milliseconds
have elapsed, all processes still existing are killed. Defaults to
infinity.
-sname Name:
Makes the Erlang runtime system into a distributed node, similar to
-name, but the host name portion of the node name Name@Host will be
the short name, not fully qualified.
This is sometimes the only way to run distributed Erlang if the
Domain Name System (DNS) is not running. No communication can exist
between nodes running with flag -sname and those running with flag
-name, as node names must be unique in distributed Erlang systems.
-start_epmd true | false:
Specifies whether Erlang should start epmd on startup. By default
this is true, but if you prefer to start epmd manually, set this to
false.
This only applies if Erlang is started as a distributed node, i.e.
if -name or -sname is specified. Otherwise, epmd is not started
even if -start_epmd true is given.
Note that a distributed node will fail to start if epmd is not run-
ning.
-smp [enable|auto|disable]:
-smp enable and -smp start the Erlang runtime system with SMP sup-
port enabled. This can fail if no runtime system with SMP support
is available. -smp auto starts the Erlang runtime system with SMP
support enabled if it is available and more than one logical pro-
cessor is detected. -smp disable starts a runtime system without
SMP support.
Note:
The runtime system with SMP support is not available on all supported
platforms. See also flag +S.
-version (emulator flag):
Makes the emulator print its version number. The same as erl +V.
EMULATOR FLAGS
erl invokes the code for the Erlang emulator (virtual machine), which
supports the following flags:
+a size:
Suggested stack size, in kilowords, for threads in the async thread
pool. Valid range is 16-8192 kilowords. The default suggested stack
size is 16 kilowords, that is, 64 kilobyte on 32-bit architectures.
This small default size has been chosen because the number of async
threads can be large. The default size is enough for drivers deliv-
ered with Erlang/OTP, but might not be large enough for other
dynamically linked-in drivers that use the driver_async() function-
ality. Notice that the value passed is only a suggestion, and it
can even be ignored on some platforms.
+A size:
Sets the number of threads in async thread pool. Valid range is
0-1024. Defaults to 10 if thread support is available.
+B [c | d | i]:
Option c makes Ctrl-C interrupt the current shell instead of invok-
ing the emulator break handler. Option d (same as specifying +B
without an extra option) disables the break handler. Option i makes
the emulator ignore any break signal.
If option c is used with oldshell on Unix, Ctrl-C will restart the
shell process rather than interrupt it.
Notice that on Windows, this flag is only applicable for werl, not
erl (oldshell). Notice also that Ctrl-Break is used instead of
Ctrl-C on Windows.
+c true | false:
Enables or disables time correction:
true:
Enables time correction. This is the default if time correction
is supported on the specific platform.
false:
Disables time correction.
For backward compatibility, the boolean value can be omitted. This
is interpreted as +c false.
+C no_time_warp | single_time_warp | multi_time_warp:
Sets time warp mode:
no_time_warp:
No time warp mode (the default)
single_time_warp:
Single time warp mode
multi_time_warp:
Multi-time warp mode
+d:
If the emulator detects an internal error (or runs out of memory),
it, by default, generates both a crash dump and a core dump. The
core dump is, however, not very useful as the content of process
heaps is destroyed by the crash dump generation.
Option +d instructs the emulator to produce only a core dump and no
crash dump if an internal error is detected.
Calling erlang:halt/1 with a string argument still produces a crash
dump. On Unix systems, sending an emulator process a SIGUSR1 signal
also forces a crash dump.
+e Number:
Sets the maximum number of ETS tables.
+ec:
Forces option compressed on all ETS tables. Only intended for test
and evaluation.
+fnl:
The virtual machine works with filenames as if they are encoded
using the ISO Latin-1 encoding, disallowing Unicode characters with
code points > 255.
For more information about Unicode filenames, see section Unicode
Filenames in the STDLIB User's Guide. Notice that this value also
applies to command-line parameters and environment variables (see
section Unicode in Enviroment and Parameters in the STDLIB User's
Guide).
+fnu[{w|i|e}]:
The virtual machine works with filenames as if they are encoded
using UTF-8 (or some other system-specific Unicode encoding). This
is the default on operating systems that enforce Unicode encoding,
that is, Windows and MacOS X.
The +fnu switch can be followed by w, i, or e to control how
wrongly encoded filenames are to be reported:
* w means that a warning is sent to the error_logger whenever a
wrongly encoded filename is "skipped" in directory listings. This
is the default.
* i means that those wrongly encoded filenames are silently
ignored.
* e means that the API function returns an error whenever a wrongly
encoded filename (or directory name) is encountered.
Notice that file:read_link/1 always returns an error if the link
points to an invalid filename.
For more information about Unicode filenames, see section Unicode
Filenames in the STDLIB User's Guide. Notice that this value also
applies to command-line parameters and environment variables (see
section Unicode in Enviroment and Parameters in the STDLIB User's
Guide).
+fna[{w|i|e}]:
Selection between +fnl and +fnu is done based on the current locale
settings in the OS. This means that if you have set your terminal
for UTF-8 encoding, the filesystem is expected to use the same
encoding for filenames. This is default on all operating systems,
except MacOS X and Windows.
The +fna switch can be followed by w, i, or e. This has effect if
the locale settings cause the behavior of +fnu to be selected; see
the description of +fnu above. If the locale settings cause the
behavior of +fnl to be selected, then w, i, or e have no effect.
For more information about Unicode filenames, see section Unicode
Filenames in the STDLIB User's Guide. Notice that this value also
applies to command-line parameters and environment variables (see
section Unicode in Enviroment and Parameters in the STDLIB User's
Guide).
+hms Size:
Sets the default heap size of processes to the size Size.
+hmbs Size:
Sets the default binary virtual heap size of processes to the size
Size.
+hmax Size:
Sets the default maximum heap size of processes to the size Size.
Defaults to 0, which means that no maximum heap size is used. For
more information, see process_flag(max_heap_size, MaxHeapSize).
+hmaxel true|false:
Sets whether to send an error logger message or not for processes
reaching the maximum heap size. Defaults to true. For more informa-
tion, see process_flag(max_heap_size, MaxHeapSize).
+hmaxk true|false:
Sets whether to kill processes reaching the maximum heap size or
not. Default to true. For more information, see
process_flag(max_heap_size, MaxHeapSize).
+hpds Size:
Sets the initial process dictionary size of processes to the size
Size.
+hmqd off_heap|on_heap:
Sets the default value for process flag message_queue_data.
Defaults to on_heap. If +hmqd is not passed, on_heap will be the
default. For more information, see process_flag(message_queue_data,
MQD).
+K true | false:
Enables or disables the kernel poll functionality if supported by
the emulator. Defaults to false (disabled). If the emulator does
not support kernel poll, and flag +K is passed to the emulator, a
warning is issued at startup.
+l:
Enables autoload tracing, displaying information while loading
code.
+L:
Prevents loading information about source filenames and line num-
bers. This saves some memory, but exceptions do not contain infor-
mation about the filenames and line numbers.
+MFlag Value:
Memory allocator-specific flags. For more information, see
erts_alloc(3).
+pc Range:
Sets the range of characters that the system considers printable in
heuristic detection of strings. This typically affects the shell,
debugger, and io:format functions (when ~tp is used in the format
string).
Two values are supported for Range:
latin1:
The default. Only characters in the ISO Latin-1 range can be con-
sidered printable. This means that a character with a code point
> 255 is never considered printable and that lists containing
such characters are displayed as lists of integers rather than
text strings by tools.
unicode:
All printable Unicode characters are considered when determining
if a list of integers is to be displayed in string syntax. This
can give unexpected results if, for example, your font does not
cover all Unicode characters.
See also io:printable_range/0 in STDLIB.
+P Number:
Sets the maximum number of simultaneously existing processes for
this system if a Number is passed as value. Valid range for Number
is [1024-134217727]
NOTE: The actual maximum chosen may be much larger than the Number
passed. Currently the runtime system often, but not always, chooses
a value that is a power of 2. This might, however, be changed in
the future. The actual value chosen can be checked by calling
erlang:system_info(process_limit).
The default value is 262144
+Q Number:
Sets the maximum number of simultaneously existing ports for this
system if a Number is passed as value. Valid range for Number is
[1024-134217727]
NOTE: The actual maximum chosen may be much larger than the actual
Number passed. Currently the runtime system often, but not always,
chooses a value that is a power of 2. This might, however, be
changed in the future. The actual value chosen can be checked by
calling erlang:system_info(port_limit).
The default value used is normally 65536. However, if the runtime
system is able to determine maximum amount of file descriptors that
it is allowed to open and this value is larger than 65536, the cho-
sen value will increased to a value larger or equal to the maximum
amount of file descriptors that can be opened.
On Windows the default value is set to 8196 because the normal OS
limitations are set higher than most machines can handle.
+R ReleaseNumber:
Sets the compatibility mode.
The distribution mechanism is not backward compatible by default.
This flag sets the emulator in compatibility mode with an earlier
Erlang/OTP release ReleaseNumber. The release number must be in the
range <current release>-2..<current release>. This limits the emu-
lator, making it possible for it to communicate with Erlang nodes
(as well as C- and Java nodes) running that earlier release.
Note:
Ensure that all nodes (Erlang-, C-, and Java nodes) of a distributed
Erlang system is of the same Erlang/OTP release, or from two differ-
ent Erlang/OTP releases X and Y, where all Y nodes have compatibility
mode X.
+r:
Forces ETS memory block to be moved on realloc.
+rg ReaderGroupsLimit:
Limits the number of reader groups used by read/write locks opti-
mized for read operations in the Erlang runtime system. By default
the reader groups limit is 64.
When the number of schedulers is less than or equal to the reader
groups limit, each scheduler has its own reader group. When the
number of schedulers is larger than the reader groups limit, sched-
ulers share reader groups. Shared reader groups degrade read lock
and read unlock performance while many reader groups degrade write
lock performance. So, the limit is a tradeoff between performance
for read operations and performance for write operations. Each
reader group consumes 64 byte in each read/write lock.
Notice that a runtime system using shared reader groups benefits
from binding schedulers to logical processors, as the reader groups
are distributed better between schedulers.
+S Schedulers:SchedulerOnline:
Sets the number of scheduler threads to create and scheduler
threads to set online when SMP support has been enabled. The maxi-
mum for both values is 1024. If the Erlang runtime system is able
to determine the number of logical processors configured and logi-
cal processors available, Schedulers defaults to logical processors
configured, and SchedulersOnline defaults to logical processors
available; otherwise the default values are 1. Schedulers can be
omitted if :SchedulerOnline is not and conversely. The number of
schedulers online can be changed at runtime through erlang:sys-
tem_flag(schedulers_online, SchedulersOnline).
If Schedulers or SchedulersOnline is specified as a negative num-
ber, the value is subtracted from the default number of logical
processors configured or logical processors available, respec-
tively.
Specifying value 0 for Schedulers or SchedulersOnline resets the
number of scheduler threads or scheduler threads online, respec-
tively, to its default value.
This option is ignored if the emulator does not have SMP support
enabled (see flag -smp).
+SP SchedulersPercentage:SchedulersOnlinePercentage:
Similar to +S but uses percentages to set the number of scheduler
threads to create, based on logical processors configured, and
scheduler threads to set online, based on logical processors avail-
able, when SMP support has been enabled. Specified values must be >
0. For example, +SP 50:25 sets the number of scheduler threads to
50% of the logical processors configured, and the number of sched-
uler threads online to 25% of the logical processors available.
SchedulersPercentage can be omitted if :SchedulersOnlinePercentage
is not and conversely. The number of schedulers online can be
changed at runtime through erlang:system_flag(schedulers_online,
SchedulersOnline).
This option interacts with +S settings. For example, on a system
with 8 logical cores configured and 8 logical cores available, the
combination of the options +S 4:4 +SP 50:25 (in either order)
results in 2 scheduler threads (50% of 4) and 1 scheduler thread
online (25% of 4).
This option is ignored if the emulator does not have SMP support
enabled (see flag -smp).
+SDcpu DirtyCPUSchedulers:DirtyCPUSchedulersOnline:
Sets the number of dirty CPU scheduler threads to create and dirty
CPU scheduler threads to set online when threading support has been
enabled. The maximum for both values is 1024, and each value is
further limited by the settings for normal schedulers:
* The number of dirty CPU scheduler threads created cannot exceed
the number of normal scheduler threads created.
* The number of dirty CPU scheduler threads online cannot exceed
the number of normal scheduler threads online.
For details, see the +S and +SP. By default, the number of dirty
CPU scheduler threads created equals the number of normal scheduler
threads created, and the number of dirty CPU scheduler threads
online equals the number of normal scheduler threads online. Dirty-
CPUSchedulers can be omitted if :DirtyCPUSchedulersOnline is not
and conversely. The number of dirty CPU schedulers online can be
changed at runtime through erlang:system_flag(dirty_cpu_sched-
ulers_online, DirtyCPUSchedulersOnline).
The amount of dirty CPU schedulers is limited by the amount of nor-
mal schedulers in order to limit the effect on processes executing
on ordinary schedulers. If the amount of dirty CPU schedulers was
allowed to be unlimited, dirty CPU bound jobs would potentially
starve normal jobs.
This option is ignored if the emulator does not have threading sup-
port enabled. This option is experimental and is supported only if
the emulator was configured and built with support for dirty sched-
ulers enabled (it is disabled by default).
+SDPcpu DirtyCPUSchedulersPercentage:DirtyCPUSchedulersOnlinePercent-
age:
Similar to +SDcpu but uses percentages to set the number of dirty
CPU scheduler threads to create and the number of dirty CPU sched-
uler threads to set online when threading support has been enabled.
Specified values must be > 0. For example, +SDPcpu 50:25 sets the
number of dirty CPU scheduler threads to 50% of the logical proces-
sors configured and the number of dirty CPU scheduler threads
online to 25% of the logical processors available. DirtyCPUSched-
ulersPercentage can be omitted if :DirtyCPUSchedulersOnlinePercent-
age is not and conversely. The number of dirty CPU schedulers
online can be changed at runtime through erlang:sys-
tem_flag(dirty_cpu_schedulers_online, DirtyCPUSchedulersOnline).
This option interacts with +SDcpu settings. For example, on a sys-
tem with 8 logical cores configured and 8 logical cores available,
the combination of the options +SDcpu 4:4 +SDPcpu 50:25 (in either
order) results in 2 dirty CPU scheduler threads (50% of 4) and 1
dirty CPU scheduler thread online (25% of 4).
This option is ignored if the emulator does not have threading sup-
port enabled. This option is experimental and is supported only if
the emulator was configured and built with support for dirty sched-
ulers enabled (it is disabled by default).
+SDio DirtyIOSchedulers:
Sets the number of dirty I/O scheduler threads to create when
threading support has been enabled. Valid range is 0-1024. By
default, the number of dirty I/O scheduler threads created is 10,
same as the default number of threads in the async thread pool.
The amount of dirty IO schedulers is not limited by the amount of
normal schedulers like the amount of dirty CPU schedulers. This
since only I/O bound work is expected to execute on dirty I/O
schedulers. If the user should schedule CPU bound jobs on dirty I/O
schedulers, these jobs might starve ordinary jobs executing on
ordinary schedulers.
This option is ignored if the emulator does not have threading sup-
port enabled. This option is experimental and is supported only if
the emulator was configured and built with support for dirty sched-
ulers enabled (it is disabled by default).
+sFlag Value:
Scheduling specific flags.
+sbt BindType:
Sets scheduler bind type.
Schedulers can also be bound using flag +stbt. The only differ-
ence between these two flags is how the following errors are han-
dled:
* Binding of schedulers is not supported on the specific plat-
form.
* No available CPU topology. That is, the runtime system was not
able to detect the CPU topology automatically, and no user-
defined CPU topology was set.
If any of these errors occur when +sbt has been passed, the run-
time system prints an error message, and refuses to start. If any
of these errors occur when +stbt has been passed, the runtime
system silently ignores the error, and start up using unbound
schedulers.
Valid BindTypes:
u:
unbound - Schedulers are not bound to logical processors, that
is, the operating system decides where the scheduler threads
execute, and when to migrate them. This is the default.
ns:
no_spread - Schedulers with close scheduler identifiers are
bound as close as possible in hardware.
ts:
thread_spread - Thread refers to hardware threads (such as
Intel's hyper-threads). Schedulers with low scheduler identi-
fiers, are bound to the first hardware thread of each core,
then schedulers with higher scheduler identifiers are bound to
the second hardware thread of each core,and so on.
ps:
processor_spread - Schedulers are spread like thread_spread,
but also over physical processor chips.
s:
spread - Schedulers are spread as much as possible.
nnts:
no_node_thread_spread - Like thread_spread, but if multiple
Non-Uniform Memory Access (NUMA) nodes exist, schedulers are
spread over one NUMA node at a time, that is, all logical pro-
cessors of one NUMA node are bound to schedulers in sequence.
nnps:
no_node_processor_spread - Like processor_spread, but if multi-
ple NUMA nodes exist, schedulers are spread over one NUMA node
at a time, that is, all logical processors of one NUMA node are
bound to schedulers in sequence.
tnnps:
thread_no_node_processor_spread - A combination of
thread_spread, and no_node_processor_spread. Schedulers are
spread over hardware threads across NUMA nodes, but schedulers
are only spread over processors internally in one NUMA node at
a time.
db:
default_bind - Binds schedulers the default way. Defaults to
thread_no_node_processor_spread (which can change in the
future).
Binding of schedulers is only supported on newer Linux, Solaris,
FreeBSD, and Windows systems.
If no CPU topology is available when flag +sbt is processed and
BindType is any other type than u, the runtime system fails to
start. CPU topology can be defined using flag +sct. Notice that
flag +sct can have to be passed before flag +sbt on the command
line (if no CPU topology has been automatically detected).
The runtime system does by default not bind schedulers to logical
processors.
Note:
If the Erlang runtime system is the only operating system process
that binds threads to logical processors, this improves the perfor-
mance of the runtime system. However, if other operating system
processes (for example another Erlang runtime system) also bind
threads to logical processors, there can be a performance penalty
instead. This performance penalty can sometimes be severe. If so,
you are advised not to bind the schedulers.
How schedulers are bound matters. For example, in situations when
there are fewer running processes than schedulers online, the
runtime system tries to migrate processes to schedulers with low
scheduler identifiers. The more the schedulers are spread over
the hardware, the more resources are available to the runtime
system in such situations.
Note:
If a scheduler fails to bind, this is often silently ignored, as it
is not always possible to verify valid logical processor identi-
fiers. If an error is reported, it is reported to the error_logger.
If you want to verify that the schedulers have bound as requested,
call erlang:system_info(scheduler_bindings).
+sbwt none|very_short|short|medium|long|very_long:
Sets scheduler busy wait threshold. Defaults to medium. The
threshold determines how long schedulers are to busy wait when
running out of work before going to sleep.
Note:
This flag can be removed or changed at any time without prior
notice.
+scl true|false:
Enables or disables scheduler compaction of load. By default
scheduler compaction of load is enabled. When enabled, load bal-
ancing strives for a load distribution, which causes as many
scheduler threads as possible to be fully loaded (that is, not
run out of work). This is accomplished by migrating load (for
example, runnable processes) into a smaller set of schedulers
when schedulers frequently run out of work. When disabled, the
frequency with which schedulers run out of work is not taken into
account by the load balancing logic.
+scl false is similar to +sub true, but +sub true also balances
scheduler utilization between schedulers.
+sct CpuTopology:
* <Id> = integer(); when 0 =< <Id> =< 65535
* <IdRange> = <Id>-<Id>
* <IdOrIdRange> = <Id> | <IdRange>
* <IdList> = <IdOrIdRange>,<IdOrIdRange> | <IdOrIdRange>
* <LogicalIds> = L<IdList>
* <ThreadIds> = T<IdList> | t<IdList>
* <CoreIds> = C<IdList> | c<IdList>
* <ProcessorIds> = P<IdList> | p<IdList>
* <NodeIds> = N<IdList> | n<IdList>
* <IdDefs> = <LogicalIds><ThreadIds><CoreIds><Proces-
sorIds><NodeIds> | <LogicalIds><ThreadIds><Cor-
eIds><NodeIds><ProcessorIds>
* CpuTopology = <IdDefs>:<IdDefs> | <IdDefs>
Sets a user-defined CPU topology. The user-defined CPU topology
overrides any automatically detected CPU topology. The CPU topol-
ogy is used when binding schedulers to logical processors.
Uppercase letters signify real identifiers and lowercase letters
signify fake identifiers only used for description of the topol-
ogy. Identifiers passed as real identifiers can be used by the
runtime system when trying to access specific hardware; if they
are incorrect the behavior is undefined. Faked logical CPU iden-
tifiers are not accepted, as there is no point in defining the
CPU topology without real logical CPU identifiers. Thread, core,
processor, and node identifiers can be omitted. If omitted, the
thread ID defaults to t0, the core ID defaults to c0, the proces-
sor ID defaults to p0, and the node ID is left undefined. Either
each logical processor must belong to only one NUMA node, or no
logical processors must belong to any NUMA nodes.
Both increasing and decreasing <IdRange>s are allowed.
NUMA node identifiers are system wide. That is, each NUMA node on
the system must have a unique identifier. Processor identifiers
are also system wide. Core identifiers are processor wide. Thread
identifiers are core wide.
The order of the identifier types implies the hierarchy of the
CPU topology. The valid orders are as follows:
* <LogicalIds><ThreadIds><CoreIds><ProcessorIds><NodeIds>, that
is, thread is part of a core that is part of a processor, which
is part of a NUMA node.
* <LogicalIds><ThreadIds><CoreIds><NodeIds><ProcessorIds>, that
is, thread is part of a core that is part of a NUMA node, which
is part of a processor.
A CPU topology can consist of both processor external, and pro-
cessor internal NUMA nodes as long as each logical processor
belongs to only one NUMA node. If <ProcessorIds> is omitted, its
default position is before <NodeIds>. That is, the default is
processor external NUMA nodes.
If a list of identifiers is used in an <IdDefs>:
* <LogicalIds> must be a list of identifiers.
* At least one other identifier type besides <LogicalIds> must
also have a list of identifiers.
* All lists of identifiers must produce the same number of iden-
tifiers.
A simple example. A single quad core processor can be described
as follows:
% erl +sct L0-3c0-3
1> erlang:system_info(cpu_topology).
[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,2}},
{core,{logical,3}}]}]
A more complicated example with two quad core processors, each
processor in its own NUMA node. The ordering of logical proces-
sors is a bit weird. This to give a better example of identifier
lists:
% erl +sct L0-1,3-2c0-3p0N0:L7,4,6-5c0-3p1N1
1> erlang:system_info(cpu_topology).
[{node,[{processor,[{core,{logical,0}},
{core,{logical,1}},
{core,{logical,3}},
{core,{logical,2}}]}]},
{node,[{processor,[{core,{logical,7}},
{core,{logical,4}},
{core,{logical,6}},
{core,{logical,5}}]}]}]
As long as real identifiers are correct, it is OK to pass a CPU
topology that is not a correct description of the CPU topology.
When used with care this can be very useful. This to trick the
emulator to bind its schedulers as you want. For example, if you
want to run multiple Erlang runtime systems on the same machine,
you want to reduce the number of schedulers used and manipulate
the CPU topology so that they bind to different logical CPUs. An
example, with two Erlang runtime systems on a quad core machine:
% erl +sct L0-3c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname one
% erl +sct L3-0c0-3 +sbt db +S3:2 -detached -noinput -noshell -sname two
In this example, each runtime system have two schedulers each
online, and all schedulers online will run on different cores. If
we change to one scheduler online on one runtime system, and
three schedulers online on the other, all schedulers online will
still run on different cores.
Notice that a faked CPU topology that does not reflect how the
real CPU topology looks like is likely to decrease the perfor-
mance of the runtime system.
For more information, see erlang:system_info(cpu_topology).
+secio true|false:
Enables or disables eager check I/O scheduling. Defaults to true.
The default was changed from false as from ERTS 7.0. The behavior
before this flag was introduced corresponds to +secio false.
The flag effects when schedulers will check for I/O operations
possible to execute, and when such I/O operations will execute.
As the parameter name implies, schedulers are more eager to check
for I/O when true is passed. This, however, also implies that
execution of outstanding I/O operation is not prioritized to the
same extent as when false is passed.
erlang:system_info(eager_check_io) returns the value of this
parameter used when starting the virtual machine.
+sfwi Interval:
Sets scheduler-forced wakeup interval. All run queues are scanned
each Interval milliseconds. While there are sleeping schedulers
in the system, one scheduler is woken for each non-empty run
queue found. Interval default to 0, meaning this feature is dis-
abled.
Note:
This feature has been introduced as a temporary workaround for
long-executing native code, and native code that does not bump
reductions properly in OTP. When these bugs have be fixed, this
flag will be removed.
+spp Bool:
Sets default scheduler hint for port parallelism. If set to true,
the virtual machine schedules port tasks when it improves paral-
lelism in the system. If set to false, the virtual machine tries
to perform port tasks immediately, improving latency at the
expense of parallelism. Default to false. The default used can be
inspected in runtime by calling erlang:system_info(port_parallel-
ism). The default can be overridden on port creation by passing
option parallelism to erlang:open_port/2.
+sss size:
Suggested stack size, in kilowords, for scheduler threads. Valid
range is 4-8192 kilowords. The default stack size is OS-depen-
dent.
+stbt BindType:
Tries to set the scheduler bind type. The same as flag +sbt
except how some errors are handled. For more information, see
+sbt.
+sub true|false:
Enables or disables scheduler utilization balancing of load. By
default scheduler utilization balancing is disabled and instead
scheduler compaction of load is enabled, which strives for a load
distribution that causes as many scheduler threads as possible to
be fully loaded (that is, not run out of work). When scheduler
utilization balancing is enabled, the system instead tries to
balance scheduler utilization between schedulers. That is, strive
for equal scheduler utilization on all schedulers.
+sub true is only supported on systems where the runtime system
detects and uses a monotonically increasing high-resolution
clock. On other systems, the runtime system fails to start.
+sub true implies +scl false. The difference between +sub true
and +scl false is that +scl false does not try to balance the
scheduler utilization.
+swct very_eager|eager|medium|lazy|very_lazy:
Sets scheduler wake cleanup threshold. Defaults to medium. Con-
trols how eager schedulers are to be requesting wakeup because of
certain cleanup operations. When a lazy setting is used, more
outstanding cleanup operations can be left undone while a sched-
uler is idling. When an eager setting is used, schedulers are
more frequently woken, potentially increasing CPU-utilization.
Note:
This flag can be removed or changed at any time without prior
notice.
+sws default|legacy:
Sets scheduler wakeup strategy. Default strategy changed in ERTS
5.10 (Erlang/OTP R16A). This strategy was known as proposal in
Erlang/OTP R15. The legacy strategy was used as default from R13
up to and including R15.
Note:
This flag can be removed or changed at any time without prior
notice.
+swt very_low|low|medium|high|very_high:
Sets scheduler wakeup threshold. Defaults to medium. The thresh-
old determines when to wake up sleeping schedulers when more work
than can be handled by currently awake schedulers exists. A low
threshold causes earlier wakeups, and a high threshold causes
later wakeups. Early wakeups distribute work over multiple sched-
ulers faster, but work does more easily bounce between sched-
ulers.
Note:
This flag can be removed or changed at any time without prior
notice.
+t size:
Sets the maximum number of atoms the virtual machine can handle.
Defaults to 1,048,576.
+T Level:
Enables modified timing and sets the modified timing level. Valid
range is 0-9. The timing of the runtime system is changed. A high
level usually means a greater change than a low level. Changing the
timing can be very useful for finding timing-related bugs.
Modified timing affects the following:
Process spawning:
A process calling spawn, spawn_link, spawn_monitor, or spawn_opt
is scheduled out immediately after completing the call. When
higher modified timing levels are used, the caller also sleeps
for a while after it is scheduled out.
Context reductions:
The number of reductions a process is allowed to use before it is
scheduled out is increased or reduced.
Input reductions:
The number of reductions performed before checking I/O is
increased or reduced.
Note:
Performance suffers when modified timing is enabled. This flag is
only intended for testing and debugging.
return_to and return_from trace messages are lost when tracing on the
spawn BIFs.
This flag can be removed or changed at any time without prior notice.
+v:
Verbose.
+V:
Makes the emulator print its version number.
+W w | i | e:
Sets the mapping of warning messages for error_logger. Messages
sent to the error logger using one of the warning routines can be
mapped to errors (+W e), warnings (+W w), or information reports
(+W i). Defaults to warnings. The current mapping can be retrieved
using error_logger:warning_map/0. For more information, see
error_logger:warning_map/0 in Kernel.
+zFlag Value:
Miscellaneous flags:
+zdbbl size:
Sets the distribution buffer busy limit (dist_buf_busy_limit) in
kilobytes. Valid range is 1-2097151. Defaults to 1024.
A larger buffer limit allows processes to buffer more outgoing
messages over the distribution. When the buffer limit has been
reached, sending processes will be suspended until the buffer
size has shrunk. The buffer limit is per distribution channel. A
higher limit gives lower latency and higher throughput at the
expense of higher memory use.
+zdntgc time:
Sets the delayed node table garbage collection time
(delayed_node_table_gc) in seconds. Valid values are either
infinity or an integer in the range 0-100000000. Defaults to 60.
Node table entries that are not referred linger in the table for
at least the amount of time that this parameter determines. The
lingering prevents repeated deletions and insertions in the
tables from occurring.
ENVIRONMENT VARIABLES
ERL_CRASH_DUMP:
If the emulator needs to write a crash dump, the value of this
variable is the filename of the crash dump file. If the variable is
not set, the name of the crash dump file is erl_crash.dump in the
current directory.
ERL_CRASH_DUMP_NICE:
Unix systems: If the emulator needs to write a crash dump, it uses
the value of this variable to set the nice value for the process,
thus lowering its priority. Valid range is 1-39 (higher values are
replaced with 39). The highest value, 39, gives the process the
lowest priority.
ERL_CRASH_DUMP_SECONDS:
Unix systems: This variable gives the number of seconds that the
emulator is allowed to spend writing a crash dump. When the given
number of seconds have elapsed, the emulator is terminated by a
SIGALRM signal.
If the variable is not set or set to 0 seconds (ERL_CRASH_DUMP_SEC-
ONDS=0), the runtime system does not even attempt to write the
crash dump file. It only terminates.
If the variable is set to negative value, such as
ERL_CRASH_DUMP_SECONDS=-1, the runtime system waits indefinitely
for the crash dump file to be written.
This variable is used with heart(3) if heart is running:
ERL_CRASH_DUMP_SECONDS=0:
Suppresses the writing a crash dump file entirely, thus rebooting
the runtime system immediately. This is the same as not setting
the environment variable.
ERL_CRASH_DUMP_SECONDS=-1:
Setting the environment variable to a negative value causes the
termination of the runtime system to wait until the crash dump
file has been completly written.
ERL_CRASH_DUMP_SECONDS=S:
Waits for S seconds to complete the crash dump file and then ter-
minates the runtime system.
ERL_CRASH_DUMP_BYTES:
This variable sets the maximum size of a crash dump file in bytes.
The crash dump will be truncated if this limit is exceeded. If the
variable is not set, no size limit is enforced by default. If the
variable is set to 0, the runtime system does not even attempt to
write a crash dump file.
Introduced in ERTS 8.1.2 (Erlang/OTP 19.2).
ERL_AFLAGS:
The content of this variable is added to the beginning of the com-
mand line for erl.
Flag -extra is treated in a special way. Its scope ends at the end
of the environment variable content. Arguments following an -extra
flag are moved on the command line into section -extra, that is,
the end of the command line following an -extra flag.
ERL_ZFLAGS and ERL_FLAGS:
The content of these variables are added to the end of the command
line for erl.
Flag -extra is treated in a special way. Its scope ends at the end
of the environment variable content. Arguments following an -extra
flag are moved on the command line into section -extra, that is,
the end of the command line following an -extra flag.
ERL_LIBS:
Contains a list of additional library directories that the code
server searches for applications and adds to the code path; see
code(3).
ERL_EPMD_ADDRESS:
Can be set to a comma-separated list of IP addresses, in which case
the epmd daemon listens only on the specified address(es) and on
the loopback address (which is implicitly added to the list if it
has not been specified).
ERL_EPMD_PORT:
Can contain the port number to use when communicating with epmd.
The default port works fine in most cases. A different port can be
specified to allow nodes of independent clusters to co-exist on the
same host. All nodes in a cluster must use the same epmd port num-
ber.
SIGNALS
On Unix systems, the Erlang runtime will interpret two types of sig-
nals.
SIGUSR1:
A SIGUSR1 signal forces a crash dump.
SIGTERM:
A SIGTERM will produce a stop message to the init process. This is
equivalent to a init:stop/0 call.
Introduced in ERTS 8.3 (Erlang/OTP 19.3)
The signal SIGUSR2 is reserved for internal usage. No other signals are
handled.
CONFIGURATION
The standard Erlang/OTP system can be reconfigured to change the
default behavior on startup.
The .erlang startup file:
When Erlang/OTP is started, the system searches for a file named
.erlang in the directory where Erlang/OTP is started. If not found,
the user's home directory is searched for an .erlang file.
If an .erlang file is found, it is assumed to contain valid Erlang
expressions. These expressions are evaluated as if they were input
to the shell.
A typical .erlang file contains a set of search paths, for example:
io:format("executing user profile in HOME/.erlang\n",[]).
code:add_path("/home/calvin/test/ebin").
code:add_path("/home/hobbes/bigappl-1.2/ebin").
io:format(".erlang rc finished\n",[]).
user_default and shell_default:
Functions in the shell that are not prefixed by a module name are
assumed to be functional objects (funs), built-in functions (BIFs),
or belong to the module user_default or shell_default.
To include private shell commands, define them in a module
user_default and add the following argument as the first line in
the .erlang file:
code:load_abs("..../user_default").
erl:
If the contents of .erlang are changed and a private version of
user_default is defined, the Erlang/OTP environment can be custom-
ized. More powerful changes can be made by supplying command-line
arguments in the startup script erl. For more information, see
init(3).
SEE ALSO
epmd(1), erl_prim_loader(3), erts_alloc(3), init(3), application(3),
auth(3), code(3), erl_boot_server(3), heart(3), net_kernel(3), make(3)
Ericsson AB erts 8.3 erl(1)