Re: Speed up Clog Access by increasing CLOG buffers

Hi,

On 2016-03-24 01:10:55 +0100, Andres Freund wrote:
> I'm afraid that this patch might be putting bandaid on some of the
> absolutely worst cases, without actually addressing the core
> problem. Simon's patch in [1] seems to come closer addressing that
> (which I don't believe it's safe without going doing every status
> manipulation atomically, as individual status bits are smaller than 4
> bytes). Now it's possibly to argue that the bandaid might slow the
> bleeding to a survivable level, but I have to admit I'm doubtful.
>
> Here's the stats for a -s 500 run btw:
> Performance counter stats for 'system wide':
> 18,747 probe_postgres:TransactionIdSetTreeStatus
> 68,884 probe_postgres:TransactionIdGetStatus
> 9,718 probe_postgres:PGSemaphoreLock
> (the PGSemaphoreLock is over 50% ProcArrayLock, followed by ~15%
> SimpleLruReadPage_ReadOnly)
>
>
> My suspicion is that a better approach for now would be to take Simon's
> patch, but add a (per-page?) 'ClogModificationLock'; to avoid the need
> of doing something fancier in TransactionIdSetStatusBit().
>
> Andres
>
> [1]: http://archives.postgresql.org/message-id/CANP8%2Bj%2BimQfHxkChFyfnXDyi6k-arAzRV%2BZG-V_OFxEtJjOL2Q%40mail.gmail.com

Simon, would you mind if I took your patch for a spin like roughly
suggested above?

Andres