Re: Speed up Clog Access by increasing CLOG buffers

On Thu, Dec 29, 2016 at 10:41 AM, Dilip Kumar <dilipbalaut(at)gmail(dot)com> wrote:
>
> I have done one more pass of the review today. I have few comments.
>
> + if (nextidx != INVALID_PGPROCNO)
> + {
> + /* Sleep until the leader updates our XID status. */
> + for (;;)
> + {
> + /* acts as a read barrier */
> + PGSemaphoreLock(&proc->sem);
> + if (!proc->clogGroupMember)
> + break;
> + extraWaits++;
> + }
> +
> + Assert(pg_atomic_read_u32(&proc->clogGroupNext) == INVALID_PGPROCNO);
> +
> + /* Fix semaphore count for any absorbed wakeups */
> + while (extraWaits-- > 0)
> + PGSemaphoreUnlock(&proc->sem);
> + return true;
> + }
>
> 1. extraWaits is used only locally in this block so I guess we can
> declare inside this block only.
>

Agreed and changed accordingly.

> 2. It seems that we have missed one unlock in case of absorbed
> wakeups. You have initialised extraWaits with -1 and if there is one
> extra wake up then extraWaits will become 0 (it means we have made one
> extra call to PGSemaphoreLock and it's our responsibility to fix it as
> the leader will Unlock only once). But it appear in such case we will
> not make any call to PGSemaphoreUnlock.
>

Good catch! I have fixed it by initialising extraWaits to 0. This
same issue exists from Group clear xid for which I will send a patch
separately.

Apart from above, the patch needs to be adjusted for commit be7b2848
which has changed the definition of PGSemaphore.

--
With Regards,
Amit Kapila.
EnterpriseDB: http://www.enterprisedb.com