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

On Mon, Jan 23, 2012 at 11:01 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Mon, Jan 23, 2012 at 12:12 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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
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