Date: Thu, 24 Mar 2016 01:14:25 +0100
From: Andres Freund <andres@anarazel.de>
To: Amit Kapila <amit.kapila16@gmail.com>,
	Jesper Pedersen <jesper.pedersen@redhat.com>,
	Simon Riggs <simon@2ndQuadrant.com>
Cc: David Steele <david@pgmasters.net>, Robert Haas <robertmhaas@gmail.com>,
	Thom Brown <thom@linux.com>, Simon Riggs <simon@2ndquadrant.com>,
	pgsql-hackers <pgsql-hackers@postgresql.org>
Subject: Re: Speed up Clog Access by increasing CLOG buffers
Message-ID: <20160324001425.GE4686@awork2.anarazel.de>
References: 
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 <CAA4eK1L4iV-2qe7AyMVsb+nz7SiX8JvCO+CqhXwaiXgm3CaBUw@mail.gmail.com>
 <CAA4eK1KZ1HVOjy39i3FRS9MsbR6GgH8D7cqyoYJxyb2PJt05Qg@mail.gmail.com>
 <56E7032D.6060908@pgmasters.net>
 <CAA4eK1LGgOyn9OpiK8W3PfrXqfHsvTi0hy0y00bu50YfE_X+MA@mail.gmail.com>
 <20160322105221.GD3790@awork2.anarazel.de>
 <CAA4eK1+8gQTyGSZLe1Rb7jeM1Beh4FqA4VNjtpZcmvwizDQ0hw@mail.gmail.com>
 <CAA4eK1LqO6wzAZ3ik7Pej__DFTgA-0GTQcXiFwnNRtQ+LmBttA@mail.gmail.com>
 <20160323204341.GB4686@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%40mail.gmail.com

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


Andres


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