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Track spinlock delay in microsecond granularity.

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On many platforms the OS will round the sleep time to millisecond
resolution, but there is no reason for us to pre-emptively round the
argument to pg_usleep.

When the delay was measured in milliseconds and started from 1 ms, it
sometimes took many attempts until the logic that increases the delay by
multiplying with a random value between 1 and 2 actually managed to bump it
from 1 ms to 2 ms. That lead to a sequence of 1 ms waits until the delay
started to increase. This wasn't really a problem but it looked odd if you
observed the waits. There is no measurable difference in performance, but
it's more readable this way.

Jeff Janes
This commit is contained in:
Heikki Linnakangas 2013-06-28 12:30:55 +03:00
parent 9db4ad44eb
commit 9e0bc7c1e8
1 changed files with 6 additions and 11 deletions
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17
src/backend/storage/lmgr/s_lock.c
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@ -81,17 +81,12 @@ s_lock(volatile slock_t *lock, const char *file, int line)
* so minutes. It seems better to fix the total number of tries (and thus * so minutes. It seems better to fix the total number of tries (and thus
* the probability of unintended failure) than to fix the total time * the probability of unintended failure) than to fix the total time
* spent. * spent.
*
* The pg_usleep() delays are measured in milliseconds because 1 msec is a
* common resolution limit at the OS level for newer platforms. On older
* platforms the resolution limit is usually 10 msec, in which case the
* total delay before timeout will be a bit more.
*/ */
#define MIN_SPINS_PER_DELAY 10 #define MIN_SPINS_PER_DELAY 10
#define MAX_SPINS_PER_DELAY 1000 #define MAX_SPINS_PER_DELAY 1000
#define NUM_DELAYS 1000 #define NUM_DELAYS 1000
#define MIN_DELAY_MSEC 1 #define MIN_DELAY_USEC 1000L
#define MAX_DELAY_MSEC 1000 #define MAX_DELAY_USEC 1000000L
int spins = 0; int spins = 0;
int delays = 0; int delays = 0;
@ -109,9 +104,9 @@ s_lock(volatile slock_t *lock, const char *file, int line)
s_lock_stuck(lock, file, line); s_lock_stuck(lock, file, line);
if (cur_delay == 0) /* first time to delay? */ if (cur_delay == 0) /* first time to delay? */
cur_delay = MIN_DELAY_MSEC; cur_delay = MIN_DELAY_USEC;
pg_usleep(cur_delay * 1000L); pg_usleep(cur_delay);
#if defined(S_LOCK_TEST) #if defined(S_LOCK_TEST)
fprintf(stdout, "*"); fprintf(stdout, "*");
@ -122,8 +117,8 @@ s_lock(volatile slock_t *lock, const char *file, int line)
cur_delay += (int) (cur_delay * cur_delay += (int) (cur_delay *
((double) random() / (double) MAX_RANDOM_VALUE) + 0.5); ((double) random() / (double) MAX_RANDOM_VALUE) + 0.5);
/* wrap back to minimum delay when max is exceeded */ /* wrap back to minimum delay when max is exceeded */
if (cur_delay > MAX_DELAY_MSEC) if (cur_delay > MAX_DELAY_USEC)
cur_delay = MIN_DELAY_MSEC; cur_delay = MIN_DELAY_USEC;
spins = 0; spins = 0;
} }