From: | Jeff Janes <jeff(dot)janes(at)gmail(dot)com> |
---|---|
To: | Peter Geoghegan <pg(at)heroku(dot)com> |
Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, "ktm(at)rice(dot)edu" <ktm(at)rice(dot)edu>, Tan Tran <tankimtran(at)gmail(dot)com>, Greg Stark <stark(at)mit(dot)edu>, pgsql-advocacy <pgsql-advocacy(at)postgresql(dot)org>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [HACKERS] GSoC on WAL-logging hash indexes |
Date: | 2014-04-30 18:10:22 |
Message-ID: | CAMkU=1xVrLq4sDxvRyvAn7sG=nQGt-PiCrf9=PYJ4nXziN3srQ@mail.gmail.com |
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On Wed, Apr 30, 2014 at 11:02 AM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:
> On Wed, Apr 30, 2014 at 10:11 AM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> > I thought the theoretical advantage of hash indexes wasn't that they
> > were smaller but that you avoided a central contention point (the
> > btree root).
>
> The B-Tree root isn't really a central contention point at all. The
> locking/latching protocol that nbtree uses is remarkably
> concurrency-friendly. In the real world, there is pretty much no
> exclusive locking of the root page's buffer.
>
I've seen the simple pinning and unpinning of the root page (or the fast
root, whatever the first page we bother to pin on a regular basis is
called) be a point of contention. When one index dominates the entire
system workload, that one page also drives contention on the spin lock that
protects the lwlock that share-protects whichever buffer mapping partition
happens to contain it.
Cheers,
Jeff
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