Horton

Re: heap vacuum & cleanup locks

Greg Stark <gsstark@mit.edu>

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From: Greg Stark <gsstark@mit.edu>
To: Simon Riggs <simon@2ndquadrant.com>
Cc: Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>, Jim Nasby <jim@nasby.net>, Robert Haas <robertmhaas@gmail.com>, Itagaki Takahiro <itagaki.takahiro@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2011-06-07T19:24:07Z
Lists: pgsql-hackers

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  1. Make VACUUM avoid waiting for a cleanup lock, where possible.

On Mon, Jun 6, 2011 at 11:30 PM, Simon Riggs <simon@2ndquadrant.com> wrote:
> But I think you've hit the important point here. The problem is not
> whether VACUUM waits for the pin, its that the pins can be held for
> extended periods.

Yes

> It makes more sense to try to limit pin hold times than it does to
> come up with pin avoidance techniques.

Well it's super-exclusive-vacuum-lock avoidance techniques. Why
shouldn't it make more sense to try to reduce the frequency and impact
of the single-purpose outlier in a non-critical-path instead of
burdening every other data reader with extra overhead?

I think Robert's plan is exactly right though I would phrase it
differently. We should get the exclusive lock, freeze/kill any xids
and line pointers, then if the pin-count is 1 do the compaction.

I'm really wishing we had more bits in the vm. It looks like we could use:
 - contains not-all-visible tuples
 - contains not-frozen xids
 - in need of compaction

I'm sure we could find a use for one more page-level vm bit too.



-- 
greg