PCTR(4) - Device Drivers Manual (i386) #

PCTR(4) - Device Drivers Manual (i386)

NAME #

pctr - driver for CPU performance counters

SYNOPSIS #

pseudo-device pctr 1

DESCRIPTION #

The pctr device provides access to the performance counters on AMD and Intel brand processors, and to the TSC on others.

Intel processors have two 40-bit performance counters which can be programmed to count events such as cache misses, branch target buffer hits, TLB misses, dual-issues, interrupts, pipeline flushes, and more. While AMD processors have four 48-bit counters, their precision is decreased to 40 bits.

There is one ioctl call to read the status of all counters, and one ioctl call to program the function of each counter. All require the following includes:

#include <sys/types.h>
#include <machine/cpu.h>
#include <machine/pctr.h>

The current state of all counters can be read with the PCIOCRD ioctl, which takes an argument of type struct pctrst:

#define PCTR_NUM	4
struct pctrst {
	u_int pctr_fn[PCTR_NUM];
	pctrval pctr_tsc;
	pctrval pctr_hwc[PCTR_NUM];
};

In this structure, ctr_fn contains the functions of the counters, as previously set by the PCIOCS0, PCIOCS1, PCIOCS2 and PCIOCS3 ioctls (see below). pctr_hwc contains the actual value of the hardware counters. pctr_tsc is a free-running, 64-bit cycle counter.

The functions of the counters can be programmed with ioctls PCIOCS0, PCIOCS1, PCIOCS2 and PCIOCS3 which require a writeable file descriptor and take an argument of type unsigned int. `` The meaning of this integer is dependent on the particular CPU.

Time stamp counter #

The time stamp counter is available on most of the AMD K6, Intel Pentium and higher class CPUs, as well as on some 486s and non-intel CPUs. It is set to zero at boot time, and then increments with each cycle. Because the counter is 64-bits wide, it does not overflow.

The time stamp counter can be read directly from user-mode using the rdtsc() macro, which returns a 64-bit value of type pctrval. The following example illustrates a simple use of rdtsc() to measure the execution time of a hypothetical subroutine called functionx():

void
time_functionx(void)
{
	pctrval tsc;

	tsc = rdtsc();
	functionx();
	tsc = rdtsc() - tsc;
	printf("Functionx took %llu cycles.\n", tsc);
}

The value of the time stamp counter is also returned by the PCIOCRD ioctl, so that one can get an exact timestamp on readings of the hardware event counters.

Intel Pentium counters #

The Intel Pentium counters are programmed with a 9 bit function. The top three bits contain the following flags:

P5CTR_K

Enables counting of events that occur in kernel mode.

P5CTR_U

Enables counting of events that occur in user mode. You must set at least one of P5CTR_U and P5CTR_K to count anything.

P5CTR_C

When this flag is set, the counter attempts to count the number of cycles spent servicing a particular event, rather than simply the number of occurrences of that event.

The bottom 6 bits set the particular event counted. A list of possible event functions could be obtained by running a pctr(1) command with -l option.

Counters for AMD K6, Intel Pentium Pro and newer CPUs #

Unlike the Pentium counters, these counters can be read directly from user-mode without need to invoke the kernel. The macro rdpmc(ctr) takes 0, 1, 2 or 3 as an argument to specify a counter, and returns that counter’s 40-bit value (which will be of type pctrval). This is generally preferable to making a system call as it introduces less distortion in measurements.

Counter functions supported by these CPUs contain several parts. The most significant byte (an 8-bit integer shifted left by PCTR_CM_SHIFT) contains a counter mask. If non-zero, this sets a threshold for the number of times an event must occur in one cycle for the counter to be incremented. The counter mask can therefore be used to count cycles in which an event occurs at least some number of times. The next byte contains several flags:

PCTR_U

Enables counting of events that occur in user mode.

PCTR_K

Enables counting of events that occur in kernel mode. You must set at least one of PCTR_K and PCTR_U to count anything.

PCTR_E

Counts edges rather than cycles. For some functions this allows you to get an estimate of the number of events rather than the number of cycles occupied by those events.

PCTR_EN

Enable counters. This bit must be set in the function for counter 0 in order for either of the counters to be enabled. This bit should probably be set in counter 1 as well.

PCTR_I

Inverts the sense of the counter mask. ‌ When this bit is set, the counter only increments on cycles in which there are no more events than specified in the counter mask.

The next byte (shifted left by the PCTR_UM_SHIFT) contains flags specific to the event being counted, also known as the unit mask.

For events dealing with the L2 cache, the following flags are valid on Intel brand processors:

PCTR_UM_M

Count events involving modified cache coherency state lines.

PCTR_UM_E

Count events involving exclusive cache coherency state lines.

PCTR_UM_S

Count events involving shared cache coherency state lines.

PCTR_UM_I

Count events involving invalid cache coherency state lines.

To measure all L2 cache activity, all these bits should be set. They can be set with the macro PCTR_UM_MESI which contains the bitwise or of all of the above.

For event types dealing with bus transactions, there is another flag that can be set in the unit mask:

PCTR_UM_A

Count all appropriate bus events, not just those initiated by the processor.

Events marked (MESI) require the PCTR_UM_[MESI] bits in the unit mask. ‌ Events marked (A) can take the PCTR_UM_A bit.

Finally, the least significant byte of the counter function is the event type to count. A list of possible event functions could be obtained by running a pctr(1) command with -l option.

FILES #

/dev/pctr

ERRORS #

[ENODEV]

An attempt was made to set the counter functions on a CPU that does not support counters.

[EINVAL]

An invalid counter function was provided as an argument to the PCIOCSx ioctl.

[EPERM]

An attempt was made to set the counter functions, but the device was not open for writing.

HISTORY #

A pctr device first appeared in OpenBSD 2.0 but was subsequently extended to support AMD and newer Intel CPUs in OpenBSD 4.3.

AUTHORS #

The pctr device was written by David Mazieres <dm@lcs.mit.edu>.

BUGS #

Not all counter functions are completely accurate. Some of the functions may not make any sense at all. Also you should be aware of the possibility of an interrupt between invocations of rdpmc() and/or rdtsc() that can potentially decrease the accuracy of measurements.

OpenBSD 7.5 - July 16, 2013