Re: Reducing contention for the LockMgrLock

Simon Riggs <simon(at)2ndquadrant(dot)com> writes:
> The output you gave wasn't anything I recognize in the code. Assuming
> its not already there, please can you share code you are using to find
> the evidence, even if its just privately in some form?

See below. Also, the message I previously mentioned shows a different
tack on the same theme:
http://archives.postgresql.org/pgsql-patches/2003-12/msg00365.php
although in the light of later events I think that keeping the counts
in shared memory like that is a bad idea --- too likely to skew the
results.

> You're looking at the number of spins to acquire each lock?

Number of semop waits.

> Manfred's earlier patch provides very clear output for observing
> contention, including full summaries. Could we commit that, so we can
> all use this for analysis? Updated with the wait info.

What patch would that be?

regards, tom lane

*** src/backend/storage/ipc/ipc.c.orig Tue Nov 22 16:06:33 2005
--- src/backend/storage/ipc/ipc.c Tue Nov 29 12:27:13 2005
***************
*** 125,130 ****
--- 125,132 ----
{
elog(DEBUG3, "shmem_exit(%d)", code);

+ LWLockStats();
+
/*
* call all the registered callbacks.
*
*** src/backend/storage/lmgr/lwlock.c.orig Tue Dec 6 18:08:33 2005
--- src/backend/storage/lmgr/lwlock.c Tue Dec 6 18:16:05 2005
***************
*** 21,26 ****
--- 21,28 ----
*/
#include "postgres.h"

+ #include <unistd.h>
+
#include "access/clog.h"
#include "access/multixact.h"
#include "access/subtrans.h"
***************
*** 32,37 ****
--- 34,43 ----
/* We use the ShmemLock spinlock to protect LWLockAssign */
extern slock_t *ShmemLock;

+ static int num_counts;
+ static int *sh_acquire_counts;
+ static int *ex_acquire_counts;
+ static int *block_counts;

typedef struct LWLock
{
***************
*** 209,214 ****
--- 215,226 ----
LWLockCounter = (int *) ((char *) LWLockArray - 2 * sizeof(int));
LWLockCounter[0] = (int) NumFixedLWLocks;
LWLockCounter[1] = numLocks;
+
+ /* local counter space */
+ num_counts = numLocks;
+ sh_acquire_counts = calloc(numLocks, sizeof(int));
+ ex_acquire_counts = calloc(numLocks, sizeof(int));
+ block_counts = calloc(numLocks, sizeof(int));
}


***************
*** 257,262 ****
--- 269,278 ----
int extraWaits = 0;

PRINT_LWDEBUG("LWLockAcquire", lockid, lock);
+ if (mode == LW_EXCLUSIVE)
+ ex_acquire_counts[lockid]++;
+ else
+ sh_acquire_counts[lockid]++;

/*
* We can't wait if we haven't got a PGPROC. This should only occur
***************
*** 328,333 ****
--- 344,351 ----
if (!mustwait)
break; /* got the lock */

+ block_counts[lockid]++;
+
/*
* Add myself to wait queue.
*
***************
*** 598,601 ****
--- 616,640 ----
return true;
}
return false;
+ }
+
+ void
+ LWLockStats(void)
+ {
+ int pid = getpid();
+ int i;
+
+ LWLockAcquire(0, LW_EXCLUSIVE);
+
+ for (i = 0; i < num_counts; i++)
+ {
+ if (sh_acquire_counts[i] || ex_acquire_counts[i] || block_counts[i])
+ {
+ fprintf(stderr, "PID %d lwlock %d: shacq %u exacq %u blk %u\n",
+ pid, i, sh_acquire_counts[i], ex_acquire_counts[i],
+ block_counts[i]);
+ }
+ }
+
+ LWLockRelease(0);
}