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From: Andres Freund <andres@anarazel.de>
To: Amit Kapila <amit.kapila16@gmail.com>
To: Jesper Pedersen <jesper.pedersen@redhat.com>
To: Simon Riggs <simon@2ndQuadrant.com>
Cc: David Steele <david@pgmasters.net>
Cc: Robert Haas <robertmhaas@gmail.com>
Cc: Thom Brown <thom@linux.com>
Cc: Simon Riggs <simon@2ndquadrant.com>
Cc: pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Speed up Clog Access by increasing CLOG buffers
Date: Thu, 24 Mar 2016 01:14:25 +0100
Message-ID: <20160324001425.GE4686@awork2.anarazel.de> (raw)
In-Reply-To: <20160324001055.GD4686@awork2.anarazel.de>
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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%...

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


Andres


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