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Re: tracking context switches with perf record

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
Cc: pgsql-hackers(at)postgresql(dot)org
Subject: Re: tracking context switches with perf record
Date: 2012-03-31 02:46:50
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Lists: pgsql-hackers
On Fri, Mar 30, 2012 at 10:07 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> Which I've now done, and it actually goes the other way.  with -F50,
> not only do almost all pgbench_accounts updates still lead to a clean,
> but a good chunk of the updates to pgbench_tellers lead to a clean as
> well.  If cleans are going to happen so often, would it make sense to
> write a combined record for clean+hot_update?

I think that'd be hard to arrange.  You might thing that the prune
happens when we go to update and find that there isn't room on the
page, but that's actually not how it works at all:

# Ideally we would do defragmenting only when we are about to attempt
# heap_update on a HOT-safe tuple.  The difficulty with this approach
# is that the update query has certainly got a pin on the old tuple, and
# therefore our attempt to acquire a buffer cleanup lock will always fail.
# (This corresponds to the idea that we don't want to move the old tuple
# out from under where the query's HeapTuple pointer points.  It might
# be possible to finesse that, but it seems fragile.)

So what we do instead is:

# The currently planned heuristic is to prune and defrag when first accessing
# a page that potentially has prunable tuples (as flagged by the pd_prune_xid
# page hint field) and that either has free space less than MAX(fillfactor
# target free space, BLCKSZ/10) *or* has recently had an UPDATE fail to
# find enough free space to store an updated tuple version.  (These rules
# are subject to change.)

So, in other words, the prune happens before we have any idea that
we're going to attempt an update.

Robert Haas
The Enterprise PostgreSQL Company

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