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Re: User concurrency thresholding: where do I look?

From: "Simon Riggs" <simon(at)2ndquadrant(dot)com>
To: "Jignesh K(dot) Shah" <J(dot)K(dot)Shah(at)Sun(dot)COM>
Cc: "Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>,"Josh Berkus" <josh(at)agliodbs(dot)com>,<pgsql-performance(at)postgresql(dot)org>,"Alvaro Herrera" <alvherre(at)commandprompt(dot)com>
Subject: Re: User concurrency thresholding: where do I look?
Date: 2007-07-26 15:51:16
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Thu, 2007-07-26 at 11:27 -0400, Jignesh K. Shah wrote:

> However at 900 Users where the big drop in throughput occurs:
> It gives a different top "consumer" of time:

>               postgres`SimpleLruReadPage+0x1ac
>               postgres`TransactionIdGetStatus+0x14
>               postgres`TransactionLogFetch+0x58

TransactionIdGetStatus doesn't directly call SimpleLruReadPage().
Presumably the compiler has been rearranging things??

Looks like you're out of clog buffers. It seems like the clog buffers
aren't big enough to hold clog pages for long enough and the SELECT FOR
SHARE processing is leaving lots of additional read locks that are
increasing the number of clog requests for older xids.

Try the enclosed patch.
  Simon Riggs

Attachment: clog_buffers.v1.test.patch
Description: text/x-patch (702 bytes)

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