Chapter 23 sysinfo Provider
The sysinfo provider makes available probes that correspond to kernel statistics classified by the name sys. Because these statistics provide the input for system monitoring utilities like mpstat(1M), the sysinfo provider enables quick exploration of observed aberrant behavior.
Probes
The sysinfo provider makes available probes that correspond to the fields in the sys named kernel statistic: a probe provided by sysinfo fires immediately before the corresponding sys value is incremented. The following example shows how to display both the names and the current values of the sys named kernel statistic using the kstat(1M) command.
$ kstat -n sys module: cpu instance: 0 name: sys class: misc bawrite 123 bread 2899 bwrite 17995 ... |
The sysinfo probes are described in Table 23–1.
Table 23–1 sysinfo Probes|
bawrite |
Probe that fires whenever a buffer is about to be asynchronously written out to a device. |
|
bread |
Probe that fires whenever a buffer is physically read from a device. bread fires after the buffer has been requested from the device, but before blocking pending its completion. |
|
bwrite |
Probe that fires whenever a buffer is about to be written out to a device, whether synchronously or asynchronously. |
|
idlethread |
Probe that fires whenever a CPU enters the idle loop. |
|
intrblk |
Probe that fires whenever an interrupt thread blocks. |
|
inv_swtch |
Probe that fires whenever a running thread is forced to involuntarily give up the CPU. |
|
lread |
Probe that fires whenever a buffer is logically read from a device. |
|
lwrite |
Probe that fires whenever a buffer is logically written to a device |
|
modload |
Probe that fires whenever a kernel module is loaded. |
|
modunload |
Probe that fires whenever a kernel module is unloaded. |
|
msg |
Probe that fires whenever a msgsnd(2) or msgrcv(2) system call is made, but before the message queue operations have been performed. |
|
mutex_adenters |
Probe that fires whenever an attempt is made to acquire an owned adaptive lock. If this probe fires, one of the lockstat provider's adaptive-block or adaptive-spin probes will also fire. See Chapter 18, lockstat Provider for details. |
|
namei |
Probe that fires whenever a name lookup is attempted in the filesystem. |
|
nthreads |
Probe that fires whenever a thread is created. |
|
phread |
Probe that fires whenever a raw I/O read is about to be performed. |
|
phwrite |
Probe that fires whenever a raw I/O write is about to be performed. |
|
procovf |
Probe that fires whenever a new process cannot be created because the system is out of process table entries. |
|
pswitch |
Probe that fires whenever a CPU switches from executing one thread to executing another. |
|
readch |
Probe that fires after each successful read, but before control is returned to the thread performing the read. A read may occur through the read(2), readv(2) or pread(2) system calls. arg0 contains the number of bytes that were successfully read. |
|
rw_rdfails |
Probe that fires whenever an attempt is made to read-lock a readers/writer when the lock is either held by a writer, or desired by a writer. If this probe fires, the lockstat provider's rw-block probe will also fire. See Chapter 18, lockstat Provider for details. |
|
rw_wrfails |
Probe that fires whenever an attempt is made to write-lock a readers/writer lock when the lock is held either by some number of readers or by another writer. If this probe fires, the lockstat provider's rw-block probe will also fire. See Chapter 18, lockstat Provider for details. |
|
sema |
Probe that fires whenever a semop(2) system call is made, but before any semaphore operations have been performed. |
|
sysexec |
Probe that fires whenever an exec(2) system call is made. |
|
sysfork |
Probe that fires whenever a fork(2) system call is made. |
|
sysread |
Probe that fires whenever a read(2), readv(2), or pread(2) system call is made. |
|
sysvfork |
Probe that fires whenever a vfork(2) system call is made. |
|
syswrite |
Probe that fires whenever a write(2), writev(2), or pwrite(2) system call is made. |
|
trap |
Probe that fires whenever a processor trap occurs. Note that some processors, in particular UltraSPARC variants, handle some light-weight traps through a mechanism that does not cause this probe to fire. |
|
ufsdirblk |
Probe that fires whenever a directory block is read from the UFS file system. See ufs(7FS) for details on UFS. |
|
ufsiget |
Probe that fires whenever an inode is retrieved. See ufs(7FS) for details on UFS. |
|
ufsinopage |
Probe that fires after an in-core inode without any associated data pages has been made available for reuse. See ufs(7FS) for details on UFS. |
|
ufsipage |
Probe that fires after an in-core inode with associated data pages has been made available for reuse. This probe fires after the associated data pages have been flushed to disk. See ufs(7FS) for details on UFS. |
|
writech |
Probe that fires after each successful write, but before control is returned to the thread performing the write. A write may occur through the write(2), writev(2) or pwrite(2) system calls. arg0 contains the number of bytes that were successfully written. |
|
xcalls |
Probe that fires whenever a cross-call is about to be made. A cross-call is the operating system's mechanism for one CPU to request immediate work of another CPU. |
Arguments
The arguments to sysinfo probes are as follows:
|
arg0 |
The value by which the statistic is to be incremented. For most probes, this argument is always 1, but for some probes this argument may take other values. |
|
arg1 |
A pointer to the current value of the statistic to be incremented. This value is a 64–bit quantity that will be incremented by the value in arg0. Dereferencing this pointer enables consumers to determine the current count of the statistic corresponding to the probe. |
|
arg2 |
A pointer to the cpu_t structure that corresponds to the CPU on which the statistic is to be incremented. This structure is defined in <sys/cpuvar.h>, but it is part of the kernel implementation and should be considered Private. |
The value of arg0 is 1 for most sysinfo probes. However, the readch and writech probes set arg0 to the number of bytes read or written, respectively. This features permits you to determine the size of reads by executable name, as shown in the following example:
# dtrace -n readch'{@[execname] = quantize(arg0)}'
dtrace: description 'readch' matched 4 probes
^C
xclock
value ------------- Distribution ------------- count
16 | 0
32 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1
64 | 0
acroread
value ------------- Distribution ------------- count
16 | 0
32 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 3
64 | 0
FvwmAuto
value ------------- Distribution ------------- count
2 | 0
4 |@@@@@@@@@@@@@ 13
8 |@@@@@@@@@@@@@@@@@@@@@ 21
16 |@@@@@ 5
32 | 0
xterm
value ------------- Distribution ------------- count
16 | 0
32 |@@@@@@@@@@@@@@@@@@@@@@@@ 19
64 |@@@@@@@@@ 7
128 |@@@@@@ 5
256 | 0
fvwm2
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@ 186
1 | 0
2 | 0
4 |@@ 51
8 | 17
16 | 0
32 |@@@@@@@@@@@@@@@@@@@@@@@@@@ 503
64 | 9
128 | 0
Xsun
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@@@ 269
1 | 0
2 | 0
4 | 2
8 |@ 31
16 |@@@@@ 128
32 |@@@@@@@ 171
64 |@ 33
128 |@@@ 85
256 |@ 24
512 | 8
1024 | 21
2048 |@ 26
4096 | 21
8192 |@@@@ 94
16384 | 0
|
The sysinfo provider sets arg2 to be a pointer to a cpu_t, a structure internal to the kernel implementation. The sysinfo probes fire on the CPU on which the statistic is being incremented. Use the cpu_id member of the cpu_t structure to determine the CPU of interest.
Example
Examine the following output from mpstat(1M):
CPU minf mjf xcal intr ithr csw icsw migr smtx srw syscl usr sys wt idl 12 90 22 5760 422 299 435 26 71 116 11 1372 5 19 17 60 13 46 18 4585 193 162 431 25 69 117 12 1039 3 17 14 66 14 33 13 3186 405 381 397 21 58 105 10 770 2 17 11 70 15 34 19 4769 109 78 417 23 57 115 13 962 3 14 14 69 16 74 16 4421 437 406 448 29 77 111 8 1020 4 23 14 59 17 51 15 4493 139 110 378 23 62 109 9 928 4 18 14 65 18 41 14 4204 494 468 360 23 56 102 9 849 4 17 12 68 19 37 14 4229 115 87 363 22 50 106 10 845 3 15 14 67 20 78 17 5170 200 169 456 26 69 108 9 1119 5 21 25 49 21 53 16 4817 78 51 394 22 56 106 9 978 4 17 22 57 22 32 13 3474 486 463 347 22 48 106 9 769 3 17 17 63 23 43 15 4572 59 34 361 21 46 102 10 947 4 15 22 59 |
From the above output, you might conclude that the xcal field seems too high, especially given the relative idleness of the system. mpstat determines the value in the xcal field by examining the xcalls field of the sys kernel statistic. This aberration can therefore be explored easily by enabling the xcalls sysinfo probe, as shown in the following example:
# dtrace -n xcalls'{@[execname] = count()}'
dtrace: description 'xcalls' matched 4 probes
^C
dtterm 1
nsrd 1
in.mpathd 2
top 3
lockd 4
java_vm 10
ksh 19
iCald.pl6+RPATH 28
nwadmin 30
fsflush 34
nsrindexd 45
in.rlogind 56
in.routed 100
dtrace 153
rpc.rstatd 246
imapd 377
sched 431
nfsd 1227
find 3767
|
The output shows where to look for the source of the cross-calls. Some number of find(1) processes are causing the majority of the cross-calls. The following D script can be used to understand the problem in further detail:
syscall:::entry
/execname == "find"/
{
self->syscall = probefunc;
self->insys = 1;
}
sysinfo:::xcalls
/execname == "find"/
{
@[self->insys ? self->syscall : "<none>"] = count();
}
syscall:::return
/self->insys/
{
self->insys = 0;
self->syscall = NULL;
}
This script uses the syscall provider to attribute cross-calls from find to a particular system call. Some cross-calls, such as those resulting from page faults, might not emanate from system calls. The script prints “<none>” in these cases. Running the script results in output similar to the following example:
# dtrace -s ./find.d dtrace: script './find.d' matched 444 probes ^C <none> 2 lstat64 2433 getdents64 14873 |
This output indicates that the majority of cross-calls induced by find are in turn induced by getdents(2) system calls. Further exploration would depend on the direction you want to explore. If you want to understand why find processes are making calls to getdents, you could write a D script to aggregate on ustack() when find induces a cross-call. If you want to understand why calls to getdents are inducing cross-calls, you could write a D script to aggregate on stack() when find induces a cross-call. Whatever your next step, the presence of the xcalls probe has enabled you to quickly discover the root cause of the unusual monitoring output.
Stability
The sysinfo provider uses DTrace's stability mechanism to describe its stabilities, as shown in the following table. For more information about the stability mechanism, see Chapter 39, Stability.
|
Element |
Name stability |
Data stability |
Dependency class |
|---|---|---|---|
|
Provider |
Evolving |
Evolving |
ISA |
|
Module |
Private |
Private |
Unknown |
|
Function |
Private |
Private |
Unknown |
|
Name |
Evolving |
Evolving |
ISA |
|
Arguments |
Private |
Private |
ISA |
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