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Re: CLOG contention

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: CLOG contention
Date: 2011-12-24 09:25:05
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Lists: pgsql-hackers
On Thu, Dec 22, 2011 at 4:20 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> You mentioned "latency" so this morning I ran pgbench with -l and
> graphed the output.  There are latency spikes every few seconds.  I'm
> attaching the overall graph as well as the graph of the last 100
> seconds, where the spikes are easier to see clearly.  Now, here's the
> problem: it seems reasonable to hypothesize that the spikes are due to
> CLOG page replacement since the frequency is at least plausibly right,
> but this is obviously not enough to prove that conclusively.  Ideas?

Thanks. That illustrates the effect I explained earlier very clearly,
so now we all know I wasn't speculating.

> Also, if it is that, what do we do about it?  I don't think any of the
> ideas proposed so far are going to help much.

If you don't like guessing, don't guess, don't think. Just measure.

Does increasing the number of buffers solve the problems you see? That
must be the first port of call - is that enough, or not? If not, we
can discuss the various ideas, write patches and measure them.

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

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