Horton

Re: measuring lwlock-related latency spikes

Greg Stark <stark@mit.edu>

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From: Greg Stark <stark@mit.edu>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>, pgsql-hackers@postgresql.org
Date: 2012-04-02T20:06:24Z
Lists: pgsql-hackers

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On Mon, Apr 2, 2012 at 8:16 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> Agreed, though I think it means the fsync is happening on a filesystem
> that causes a full system fsync. That time is not normal.

I don't know what you mean. It looks like there are two cases where
this code path executes. Either more than 16 clog files are being
flushed by the SimpleLRUFlush() during a checkpoint or a dirty page is
being evicted by SlruSelectLRUPage().

I don't know that 16 is so crazy a number of clog files to be touching
between checkpoints any more on a big machine like this. The number of
clog files active concurrently in pgbench should be related to how
quickly xids are being used up and how large the database is -- both
of which are pretty big in these tests.  Perhaps the 16 should have
been raised to 32 when CLOGShmemBuffers was raised to 32.

-- 
greg