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Reimplement clock(3) using clock_gettime(3)

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Unlike times(), clock_gettime() is implemented as a vDSO on many architectures.
So, using clock_gettime() will return a more accurate time and do so with less
overhead because it does have the overhead of calling into the kernel.

It is also significantly more accurate because it measures the actual time in
nanoseconds rather than the number of ticks (typically 1 millisecond or more).

Bug: 17814435
Change-Id: Id4945d9f387330518f78669809639952e9227ed9
This commit is contained in:
Alex Van Brunt 2014-09-26 13:32:47 -07:00 committed by Elliott Hughes
parent d5df4124bb
commit 8d0b2dbf21
2 changed files with 15 additions and 6 deletions
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13
libc/bionic/clock.cpp
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@ -30,12 +30,13 @@
#include <sys/sysconf.h>
#include <sys/times.h>
#include "private/bionic_constants.h"
// http://pubs.opengroup.org/onlinepubs/9699919799/functions/clock.html
clock_t clock() {
tms t;
times(&t);
// Although times(2) and clock(3) both use the type clock_t, the units are
// different. For times(2) it's pure clock ticks, but for clock(3) the unit
// is CLOCKS_PER_SEC, so we need to scale appropriately.
return (t.tms_utime + t.tms_stime) * (CLOCKS_PER_SEC / sysconf(_SC_CLK_TCK));
timespec ts;
if (clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &ts) == -1) {
return -1;
}
return (ts.tv_sec * CLOCKS_PER_SEC) + (ts.tv_nsec / (NS_PER_S / CLOCKS_PER_SEC));
}

8
tests/time_test.cpp
View File

@ -404,3 +404,11 @@ TEST(time, clock_gettime) {
ASSERT_EQ(0, ts2.tv_sec);
ASSERT_LT(ts2.tv_nsec, 1000000);
}
TEST(time, clock) {
// clock(3) is hard to test, but a 1s sleep should cost less than 1ms.
clock_t t0 = clock();
sleep(1);
clock_t t1 = clock();
ASSERT_LT(t1 - t0, CLOCKS_PER_SEC / 1000);
}