man pages section 3: Extended Library Functions, Volume 1

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Updated: July 2014
 
 

cpc_buf_sub(3CPC)

Name

cpc_buf_create , cpc_buf_destroy , cpc_set_sample , cpc_buf_get , cpc_buf_set , cpc_buf_hrtime , cpc_buf_tick , cpc_buf_sub , cpc_buf_add , cpc_buf_copy , cpc_buf_zero , cpc_buf_smpl_rec_count , cpc_buf_smpl_get_item , cpc_buf_smpl_get_record , cpc_get_smpl_max_rec_count - sample and manipulate CPC and SMPL data

Synopsis

cc [ flag… ] file
–lcpc [ library… ] 
#include <libcpc.h>

cpc_buf_t *cpc_buf_create(
cpc_t *cpc, cpc_set_t *
set);
int cpc_buf_destroy(
cpc_t *cpc, cpc_buf_t *
buf);
int cpc_set_sample(
cpc_t *cpc, cpc_set_t *
set, cpc_buf_t *buf);
int cpc_buf_get(
cpc_t *cpc, cpc_buf_t *
buf, int index, 
uint64_t *val);
int cpc_buf_set(
cpc_t *cpc, cpc_buf_t *
buf, int index, 
uint64_t val);
hrtime_t cpc_buf_hrtime(
cpc_t *cpc, cpc_buf_t *
buf);
uint64_t cpc_buf_tick(
cpc_t *cpc, cpc_buf_t *
buf);
void cpc_buf_sub(
cpc_t *cpc, cpc_buf_t *
ds, cpc_buf_t *a, 
cpc_buf_t *b);
void cpc_buf_add(
cpc_t *cpc, cpc_buf_t *
ds, cpc_buf_t *a, 
cpc_buf_t *b);
void cpc_buf_copy(
cpc_t *cpc, cpc_buf_t *
ds, cpc_buf_t *src);
void cpc_buf_zero(
cpc_t *cpc, cpc_buf_t *
buf);
int cpc_buf_smpl_rec_count(cpc_t 
*cpc, cpc_buf_t *
buf, 
		int request_index, uint_t
 *rec_count);
int cpc_buf_smpl_get_item(cpc_t *
cpc, cpc_buf_t *
buf, 
		int request_index, int
 record_index, int 
record_item_index, uint64_t *val
);
uint64_t *cpc_buf_smpl_get_record
(cpc_t *cpc, 
cpc_buf_t *buf, 
		int request_index, 
int record_index);

Description

Counter data and sampling results are sampled into CPC buffers, which are represented by the opaque data type cpc_buf_t. A CPC buffer is created with cpc_buf_create() to hold the data for a specific CPC set. Once a CPC buffer has been created, it can only be used to store and manipulate the data of the CPC set for which it was created.

Once a set has been successfully bound, the counter values and the sampling results are retrieved using cpc_set_sample(). For the CPC request, the cpc_set_sample () function takes a snapshot of the hardware performance counters counting on behalf of the CPC requests in set and stores the 64-bit virtualized software representations of the counters in the supplied CPC buffer. For the SMPL request, the cpc_set_sample() function retrieves the sampling results collected by the hardware on behalf of the SMPL requests in set and stores the results in the supplied CPC buffer as a form of SMPL records. If a set was bound with cpc_bind_curlwp(3CPC) or cpc_bind_curlwp(3CPC), the set can only be sampled by the LWP that bound it.

The kernel maintains 64-bit virtual software counters to hold the counts accumulated for each CPC request in the set, thereby allowing applications to count past the limits of the underlying physical counter, which can be significantly smaller than 64 bits. The kernel attempts to maintain the full 64-bit counter values even in the face of physical counter overflow on architectures and processors that can automatically detect overflow. If the processor is not capable of overflow detection, the caller must ensure that the counters are sampled often enough to avoid the physical counters wrapping. The events most prone to wrap are those that count processor clock cycles. If such an event is of interest, sampling should occur frequently so that the counter does not wrap between samples.

The cpc_buf_get() function retrieves the last sampled value of a particular CPC request in buf. The index argument specifies which CPC request value in the set to retrieve. The index for each request is returned during set configuration by cpc_set_add_request(3CPC). The 64-bit virtualized software counter value is stored in the location pointed to by the val argument. If index does not specify the CPC request but the SMPL request in buf, cpc_buf_get() function sets errno to indicate error and returns -1. To retrieve the results for SMPL requests in buf, cpc_buf_smpl_get_item() and cpc_buf_smpl_get_record() should be used.

The cpc_buf_set() function stores a 64-bit value to a specific CPC request in the supplied buffer. This operation can be useful for performing calculations with CPC buffers, but it does not affect the value of the hardware counter (and thus will not affect the next sample). If index does not specify the CPC request but the SMPL request in buf, the cpc_buf_set() function sets errno to indicate error and returns -1. The operation to store values to a specific SMPL request in the specified buffer is not supported.

The cpc_buf_hrtime() function returns a high-resolution timestamp indicating exactly when the set was last sampled by the kernel.

The cpc_buf_tick() function returns a 64-bit virtualized cycle counter indicating how long the set has been programmed into the counter since it was bound. The units of the values returned by cpc_buf_tick() are CPU clock cycles.

The cpc_buf_sub() function calculates the difference between each CPC request in sets a and b, storing the result in the corresponding request within set ds. More specifically, for each CPC request index n, this function performs ds[ n] = a[n] - b[n]. Similarly, cpc_buf_add() adds each CPC request in sets a and b and stores the result in the corresponding request within set ds. If the specified buffer a and b also contain the results of the SMPL requests, cpc_buf_sub() and cpc_buf_add () skip the results of the SMPL requests and handle only the results of the CPC requests.

The cpc_buf_copy() function copies each value from buffer src into buffer ds. Both buffers must have been created from the same cpc_set_t.

The cpc_buf_zero() function sets each request's value in the buffer to zero.

The cpc_buf_destroy() function frees all resources associated with the CPC buffer.

The cpc_buf_smpl_rec_count() function returns the number of available SMPL records that are found in buf, for the SMPL request specified by request_index, into the uint_t object pointed to by rec_count .

The cpc_buf_smpl_get_item() function retrieves one record item specified by record_item_index in the SMPL record specified by record_index for the SMPL request specified by request_index, into the uint64_t object pointed to by val. record_item_index is the index to an item found in the SMPL record, which should be obtained by using cpc_walk_smpl_recitems () and cpc_walk_smpl_recitems_req(). Also, the following special macros can be used to specify generic record items for record_item_index:

SMPL_REC_ITEM_PC

Index to the program counter associated with he monitoring event; for example, this will be internally mapped to the index to the RIP item on Intel PEBS.

SMPL_REC_ITEM_DLINA

Index to the data linear address associated with the monitoring event; for example, this will be internally mapped to the index to the data linear address item on Intel PEBS.

SMPL_REC_ITEM_LAT

Index to the latency value associated with the monitoring event; for example, this will be internally mapped to the index to the latency value item on Intel PEBS.

SMPL_REC_ITEM_DSRC

Index to the data source description associated with the monitoring event; for example, this will be internally mapped to the index to the data source encoding item on Intel PEBS.

Calling cpc_buf_smpl_get_item() will not invalidate the sampling results in buf.

The cpc_buf_smpl_get_record() function returns on SMPL record specified by record_index for the SMPL request specified by request_index, as the pointer to an array of uint64_t objects. Each record item in the SMPL record should be accessed using the index to the desired SMPL record item, obtained by using cpc_walk_smpl_recitems () and cpc_walk_smpl_recitems_req(). The application must not alter the array of uint64_t objects returned by cpc_buf_smpl_get_record(). Calling cpc_buf_smpl_get_record() will not invalidate the sampling results in buf.

Return Values

Upon successful completion, cpc_buf_create() returns a pointer to a CPC buffer which can be used to hold data for the set argument. Otherwise, this function returns NULL and sets errno to indicate the error.

Upon successful completion, cpc_set_sample(), cpc_buf_get(), and cpc_buf_set() return 0. Otherwise, they return -1 and set errno to indicate the error.

cpc_buf_smpl_get_item () function sets errno to EINVAL and returns -1 if the system does not support the SMPL record item corresponding to the special macro. If cpc was not successfully opened for SMPL, or if the request specified by request_index is not for SMPL, or if record_index does not point to a record entry available in buf, or if record_item_index does not point to an available record item, the cpc_buf_smpl_get_item() function sets errno to EINVAL and returns -1.

cpc_buf_smpl_get_record() function sets errno to EINVAL and returns -1 if cpc was not successfully opened for SMPL, or if the request specified by request_index is not for SMPL, or if record_index does not point to a record entry available in buf.

Errors

These functions will fail if:

EINVAL

For cpc_set_sample(), the set is not bound, the set and/or CPC buffer were not created with the given cpc handle, or the CPC buffer was not created with the supplied set.

For cpc_buf_get() and cpc_buf_set() functions, the request specified by index is not for CPC but for SMPL.

For cpc_buf_smpl_rec_count() functions, the request specified by request_index is not for SMPL.

For cpc_buf_smpl_get_item(), the system does not support the SMPL record item corresponding to the specified special macro, cpc was not successfully opened for SMPL, the request specified by request_index is not for SMPL, record_index does not point to a record entry available in buf, or record_item_index does not point to an available record item.

For cpc_buf_smpl_get_record(), cpc was not successfully opened for SMPL, the request specified by request_index is not for SMPL, or record_index does not point to a record entry available in buf.

EAGAIN

When using cpc_set_sample() to sample a CPU-bound set, the LWP has been unbound from the processor it is measuring.

ENOMEM

The library could not allocate enough memory for its internal data structures.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Safe

See Also

cpc_bind_curlwp(3CPC), cpc_set_add_request(3CPC), libcpc(3LIB), attributes(5)

Notes

Often the overhead of performing a system call can be too disruptive to the events being measured. Once a cpc_bind_curlwp(3CPC) call has been issued, it is possible to access directly the performance hardware registers from within the application. If the performance counter context is active, the counters will count on behalf of the current LWP.

Not all processors support this type of access. On processors where direct access is not possible, cpc_set_sample() must be used to read the counters.

SPARC
rd %pic, %rN        ! All UltraSPARC
wr %rN, %pic        ! (All UltraSPARC, but see text)
x86
rdpmc               ! Pentium II, III, and 4 only

If the counter context is not active or has been invalidated, the %pic register (SPARC), and the rdpmc instruction (Pentium) becomes unavailable.

Pentium II and III processors support the non-privileged rdpmc instruction that requires that the counter of interest be specified in %ecx and return a 40-bit value in the %edx :%eax register pair. There is no non-privileged access mechanism for Pentium I processors.