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15.2. POSIX Clocks

POSIX is a standard for implementing and representing time sources. In contrast to the hardware clock, which is selected by the kernel and implemented across the system; the POSIX clock can be selected by each application, without affecting other applications in the system.
  • CLOCK_REALTIME: it represents the time in the real world, also referred to as 'wall time' meaning the time as read from the clock on the wall. This clock is used to timestamp events, and when interfacing with the user. It can be modified by an user with the right privileges. However, user modification should be used with caution as it can lead to erroneous data if the clock has its value changed between two readings.
  • CLOCK_MONOTONIC: represents the time monotonically increased since the system boot. This clock cannot be set by any process, and is the preferred clock for calculating the time difference between events. The following examples in this section use CLOCK_MONOTONIC as the POSIX clock.

Note

For more information on POSIX clocks see the following man page and book:
  • clock_gettime()
  • Linux System Programming by Robert Love
The function used to read a given POSIX clock is clock_gettime(), which is defined at <time.h>. The clock_gettime() command takes two parameters: the POSIX clock ID and a timespec structure which will be filled with the duration used to read the clock. The following example shows the function to measure the cost of reading the clock:

Example 15.2. Using clock_gettime() to Measure the Cost of Reading POSIX Clocks

#include <time.h>

main()
{
	int rc;
	long i;
	struct timespec ts;

	for(i=0; i<10000000; i++) {
		rc = clock_gettime(CLOCK_MONOTONIC, &ts);
	}
}
You can improve upon the example above by adding more code to verify the return code of clock_gettime(), to verify the value of the rc variable, or to ensure the content of the ts structure is to be trusted. The clock_gettime() manpage provides more information to help you write more reliable applications.

Important

Programs using the clock_gettime() function must be linked with the rt library by adding '-lrt' to the gcc command line: 
~]$ gcc clock_timing.c -o clock_timing -lrt

15.2.1. CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE
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Functions such as clock_gettime() and gettimeofday() have a counterpart in the kernel, in the form of a system call. When a user process calls clock_gettime(), the corresponding C library (glibc) routine calls the sys_clock_gettime() system call, which performs the requested operation and then returns the result to the user process.
However, this context switch from user application to kernel has a cost. Even though this cost is very low, if the operation is repeated thousands of times, the accumulated cost can have an impact on the overall performance of the application.
To avoid the context switch to the kernel, thus making it faster to read the clock, support for the CLOCK_MONOTONIC_COARSE and CLOCK_REALTIME_COARSE POSIX clocks was created in the form of a VDSO library function. The _COARSE variants are faster to read and have a precision (also known as resolution) of one millisecond (ms).

15.2.2. Using clock_getres() to Compare Clock Resolution
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Using the clock_getres() function you can check the resolution of a given POSIX clock. clock_getres() uses the same two parameters as clock_gettime(): the ID of the POSIX clock to be used, and a pointer to the timespec structure where the result is returned. The following function enables you to compare the precision between CLOCK_MONOTONIC and CLOCK_MONOTONIC_COARSE: 
main()
{
int rc;
struct timespec res;

	rc = clock_getres(CLOCK_MONOTONIC, &res);
	if (!rc)
		printf("CLOCK_MONOTONIC: %ldns\n", res.tv_nsec); 
	rc = clock_getres(CLOCK_MONOTONIC_COARSE, &res);
	if (!rc)
		printf("CLOCK_MONOTONIC_COARSE: %ldns\n", res.tv_nsec); 
}

Example 15.3. Sample Output of clock_getres

TSC:
	~]# ./clock_resolution
	CLOCK_MONOTONIC: 1ns
	CLOCK_MONOTONIC_COARSE: 999848ns  (about 1ms)

HPET:
	~]# ./clock_resolution
	CLOCK_MONOTONIC: 1ns
	CLOCK_MONOTONIC_COARSE: 999848ns  (about 1ms)

ACPI_PM:
	~]# ./clock_resolution
	CLOCK_MONOTONIC: 1ns
	CLOCK_MONOTONIC_COARSE: 999848ns  (about 1ms)

15.2.3. Using C Code to Compare Clock Resolution
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Using the following code snippet it is possible to observe the format of the data read from the CLOCK_MONOTONIC POSIX clock. All nine digits in the tv_nsec field of the timespec structure are meaningful as the clock has a nanosecond resolution. The example function, named clock_test.c, is as follows: 
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

main()
{
	int i;
	struct timespec ts;

	for(i=0; i<5; i++) {
		clock_gettime(CLOCK_MONOTONIC, &ts);
		printf("%ld.%ld\n", ts.tv_sec, ts.tv_nsec);
		usleep(200);
	}
}

Example 15.4. Sample Output of clock_test.c and clock_test_coarse.c

As specified in the code above, the function reads the clock five times, with 200 microseconds between each reading: 
~]# gcc clock_test.c -o clock_test -lrt
~]# ./clock_test
218449.986980853
218449.987330908
218449.987590716
218449.987849549
218449.988108248
Using the same source code, renaming it to clock_test_coarse.c and replacing CLOCK_MONOTONIC with CLOCK_MONOTONIC_COARSE, the result would look something like: 
~]# ./clock_test_coarse
218550.844862154
218550.844862154
218550.844862154
218550.845862154
218550.845862154
The _COARSE clocks have a one millisecond precision, therefore only the first three digits of the tv_nsec field of the timespec structure are significant. The result above could be read as: 
~]# ./clock_test_coarse
218550.844
218550.844
218550.844
218550.845
218550.845
The _COARSE variants of the POSIX clocks are particularly useful in cases where timestamping can be performed with millisecond precision. The benefits are more evident on systems which use hardware clocks with high costs for the reading operations, such as ACPI_PM.
Using the time command to read the clock source 10 million times in a row, you can compare the costs of reading CLOCK_MONOTONIC and CLOCK_MONOTONIC_COARSE representations of the hardware clocks available. The following example uses TSC, HPET and ACPI_PM hardware clocks. For more information on how to decipher the output of the time command see Section 15.1.1, “Reading Hardware Clock Sources”.

Example 15.5. Comparing the Cost of Reading POSIX Clocks

TSC:
	~]# time ./clock_timing_monotonic

	real	0m0.567s
	user	0m0.559s
	sys	0m0.002s

	~]# time ./clock_timing_monotonic_coarse

	real	0m0.120s
	user	0m0.118s
	sys	0m0.001s

HPET:
	~]# time ./clock_timing_monotonic

	real	0m12.257s
	user	0m12.179s
	sys	0m0.002s

	~]# time ./clock_timing_monotonic_coarse

	real	0m0.119s
	user	0m0.118s
	sys	0m0.000s

ACPI_PM:
	~]# time ./clock_timing_monotonic

	real	0m25.524s
	user	0m0.451s
	sys	0m24.932s

	~]# time ./clock_timing_monotonic_coarse

	real	0m0.119s
	user	0m0.117s
	sys	0m0.001s
As seen from Example 15.5, “Comparing the Cost of Reading POSIX Clocks”, the sys time (the time spent by the kernel to perform tasks required by the user process) is greatly reduced when the _COARSE clocks are used. This is particularly evident in the ACPI_PM clock timings, which indicates that _COARSE variants of POSIX clocks yield high performance gains on clocks with high reading costs.
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