Horton

Re: Removing freelist (was Re: Should I implement DROP INDEX CONCURRENTLY?)

Robert Haas <robertmhaas@gmail.com>

View thread
From: Robert Haas <robertmhaas@gmail.com>
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Jim Nasby <jim@nasby.net>, Heikki Linnakangas <heikki.linnakangas@enterprisedb.com>, Simon Riggs <simon@2ndquadrant.com>, Daniel Farina <daniel@heroku.com>, Merlin Moncure <mmoncure@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2012-01-23T16:03:28Z
Lists: pgsql-hackers 
On Mon, Jan 23, 2012 at 11:01 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Mon, Jan 23, 2012 at 12:12 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>>> The expensive part of what
>>>> we do while holding BufFreelistLock is, I think, iterating through
>>>> buffers taking and releasing a spinlock on each one (!).
>
>>> Yeah ... spinlocks that, by definition, will be uncontested.
>
>> What makes you think that they are uncontested?
>
> Ah, never mind.  I was thinking that we'd only be touching buffers that
> were *on* the freelist, but of course this is incorrect.  The real
> problem there is that BufFreelistLock is also used to protect the
> clock sweep pointer.  I think basically we gotta find a way to allow
> multiple backends to run clock sweeps concurrently.  Or else fix
> things so that the freelist never (well, hardly ever) runs dry.

I'd come to the same conclusion myself.  :-)

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company