Header And Logo
|
#include "postgres.h"#include <time.h>#include <unistd.h>#include "storage/s_lock.h"
Go to the source code of this file.
Defines | |
| #define | MIN_SPINS_PER_DELAY 10 |
| #define | MAX_SPINS_PER_DELAY 1000 |
| #define | NUM_DELAYS 1000 |
| #define | MIN_DELAY_MSEC 1 |
| #define | MAX_DELAY_MSEC 1000 |
Functions | |
| static void | s_lock_stuck (volatile slock_t *lock, const char *file, int line) |
| int | s_lock (volatile slock_t *lock, const char *file, int line) |
| void | set_spins_per_delay (int shared_spins_per_delay) |
| int | update_spins_per_delay (int shared_spins_per_delay) |
Variables | |
| slock_t | dummy_spinlock |
| static int | spins_per_delay = DEFAULT_SPINS_PER_DELAY |
| #define MAX_DELAY_MSEC 1000 |
Referenced by s_lock().
| #define MAX_SPINS_PER_DELAY 1000 |
Referenced by s_lock().
| #define MIN_DELAY_MSEC 1 |
| #define MIN_SPINS_PER_DELAY 10 |
Referenced by s_lock().
| #define NUM_DELAYS 1000 |
Referenced by s_lock().
| int s_lock | ( | volatile slock_t * | lock, | |
| const char * | file, | |||
| int | line | |||
| ) |
Definition at line 50 of file s_lock.c.
References Max, MAX_DELAY_MSEC, MAX_RANDOM_VALUE, MAX_SPINS_PER_DELAY, Min, MIN_SPINS_PER_DELAY, NUM_DELAYS, pg_usleep(), random(), s_lock_stuck(), SPIN_DELAY, spins_per_delay, and TAS_SPIN.
{
/*
* We loop tightly for awhile, then delay using pg_usleep() and try again.
* Preferably, "awhile" should be a small multiple of the maximum time we
* expect a spinlock to be held. 100 iterations seems about right as an
* initial guess. However, on a uniprocessor the loop is a waste of
* cycles, while in a multi-CPU scenario it's usually better to spin a bit
* longer than to call the kernel, so we try to adapt the spin loop count
* depending on whether we seem to be in a uniprocessor or multiprocessor.
*
* Note: you might think MIN_SPINS_PER_DELAY should be just 1, but you'd
* be wrong; there are platforms where that can result in a "stuck
* spinlock" failure. This has been seen particularly on Alphas; it seems
* that the first TAS after returning from kernel space will always fail
* on that hardware.
*
* Once we do decide to block, we use randomly increasing pg_usleep()
* delays. The first delay is 1 msec, then the delay randomly increases to
* about one second, after which we reset to 1 msec and start again. The
* idea here is that in the presence of heavy contention we need to
* increase the delay, else the spinlock holder may never get to run and
* release the lock. (Consider situation where spinlock holder has been
* nice'd down in priority by the scheduler --- it will not get scheduled
* until all would-be acquirers are sleeping, so if we always use a 1-msec
* sleep, there is a real possibility of starvation.) But we can't just
* clamp the delay to an upper bound, else it would take a long time to
* make a reasonable number of tries.
*
* We time out and declare error after NUM_DELAYS delays (thus, exactly
* that many tries). With the given settings, this will usually take 2 or
* 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
* 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 MAX_SPINS_PER_DELAY 1000
#define NUM_DELAYS 1000
#define MIN_DELAY_MSEC 1
#define MAX_DELAY_MSEC 1000
int spins = 0;
int delays = 0;
int cur_delay = 0;
while (TAS_SPIN(lock))
{
/* CPU-specific delay each time through the loop */
SPIN_DELAY();
/* Block the process every spins_per_delay tries */
if (++spins >= spins_per_delay)
{
if (++delays > NUM_DELAYS)
s_lock_stuck(lock, file, line);
if (cur_delay == 0) /* first time to delay? */
cur_delay = MIN_DELAY_MSEC;
pg_usleep(cur_delay * 1000L);
#if defined(S_LOCK_TEST)
fprintf(stdout, "*");
fflush(stdout);
#endif
/* increase delay by a random fraction between 1X and 2X */
cur_delay += (int) (cur_delay *
((double) random() / (double) MAX_RANDOM_VALUE) + 0.5);
/* wrap back to minimum delay when max is exceeded */
if (cur_delay > MAX_DELAY_MSEC)
cur_delay = MIN_DELAY_MSEC;
spins = 0;
}
}
/*
* If we were able to acquire the lock without delaying, it's a good
* indication we are in a multiprocessor. If we had to delay, it's a sign
* (but not a sure thing) that we are in a uniprocessor. Hence, we
* decrement spins_per_delay slowly when we had to delay, and increase it
* rapidly when we didn't. It's expected that spins_per_delay will
* converge to the minimum value on a uniprocessor and to the maximum
* value on a multiprocessor.
*
* Note: spins_per_delay is local within our current process. We want to
* average these observations across multiple backends, since it's
* relatively rare for this function to even get entered, and so a single
* backend might not live long enough to converge on a good value. That
* is handled by the two routines below.
*/
if (cur_delay == 0)
{
/* we never had to delay */
if (spins_per_delay < MAX_SPINS_PER_DELAY)
spins_per_delay = Min(spins_per_delay + 100, MAX_SPINS_PER_DELAY);
}
else
{
if (spins_per_delay > MIN_SPINS_PER_DELAY)
spins_per_delay = Max(spins_per_delay - 1, MIN_SPINS_PER_DELAY);
}
return delays;
}
| static void s_lock_stuck | ( | volatile slock_t * | lock, | |
| const char * | file, | |||
| int | line | |||
| ) | [static] |
| void set_spins_per_delay | ( | int | shared_spins_per_delay | ) |
Definition at line 168 of file s_lock.c.
References spins_per_delay.
Referenced by InitAuxiliaryProcess(), and InitProcess().
{
spins_per_delay = shared_spins_per_delay;
}
| int update_spins_per_delay | ( | int | shared_spins_per_delay | ) |
Definition at line 179 of file s_lock.c.
References spins_per_delay.
Referenced by AuxiliaryProcKill(), and ProcKill().
{
/*
* We use an exponential moving average with a relatively slow adaption
* rate, so that noise in any one backend's result won't affect the shared
* value too much. As long as both inputs are within the allowed range,
* the result must be too, so we need not worry about clamping the result.
*
* We deliberately truncate rather than rounding; this is so that single
* adjustments inside a backend can affect the shared estimate (see the
* asymmetric adjustment rules above).
*/
return (shared_spins_per_delay * 15 + spins_per_delay) / 16;
}
int spins_per_delay = DEFAULT_SPINS_PER_DELAY [static] |
Definition at line 25 of file s_lock.c.
Referenced by s_lock(), set_spins_per_delay(), and update_spins_per_delay().
1.7.1