Horton

Re: Speed up Clog Access by increasing CLOG buffers

Amit Kapila <amit.kapila16@gmail.com>

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From: Amit Kapila <amit.kapila16@gmail.com>
To: Dilip Kumar <dilipbalaut@gmail.com>
Cc: Tomas Vondra <tomas.vondra@2ndquadrant.com>, Robert Haas <robertmhaas@gmail.com>, Jim Nasby <Jim.Nasby@bluetreble.com>, Andres Freund <andres@anarazel.de>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2016-12-31T03:31:05Z
Lists: pgsql-hackers

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On Thu, Dec 29, 2016 at 10:41 AM, Dilip Kumar <dilipbalaut@gmail.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

Commits

  1. Use group updates when setting transaction status in clog.

  2. Improve 64bit atomics support.

  3. Add ProcArrayGroupUpdate wait event.

  4. Make the different Unix-y semaphore implementations ABI-compatible.

  5. Fix broken ALTER INDEX documentation

  6. Code and docs review for commit 3187d6de0e5a9e805b27c48437897e8c39071d45.

  7. Partition the freelist for shared dynahash tables.

  8. Correct StartupSUBTRANS for page wraparound

  9. Make idle backends exit if the postmaster dies.

  10. contrib/sslinfo: add ssl_extension_info SRF

  11. Reduce ProcArrayLock contention by removing backends in batches.

  12. Fix `make installcheck` for serializable transactions.

  13. Lockless StrategyGetBuffer clock sweep hot path.

  14. Reduce sinval synchronization overhead.