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Re: Pooling in Core WAS: Need help in performance tuning.

From: Robert Haas <robertmhaas(at)gmail(dot)com>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Andres Freund <andres(at)anarazel(dot)de>, Craig Ringer <craig(at)postnewspapers(dot)com(dot)au>, Matthew Wakeling <matthew(at)flymine(dot)org>, pgsql-performance(at)postgresql(dot)org
Subject: Re: Pooling in Core WAS: Need help in performance tuning.
Date: 2010-07-28 01:44:40
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
On Tue, Jul 27, 2010 at 4:40 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Robert Haas <robertmhaas(at)gmail(dot)com> writes:
>> On Thu, Jul 22, 2010 at 5:29 PM, Andres Freund <andres(at)anarazel(dot)de> wrote:
>>>> The problem is harder for us because a backend can't switch identities
>>>> once it's been assigned to a database.  I haven't heard an adequate
>>>> explanation of why that couldn't be changed, though.
>>> Possibly it might decrease the performance significantly enough by
>>> reducing the cache locality (syscache, prepared plans)?
>> Those things are backend-local.  The worst case scenario is you've got
>> to flush them all when you reinitialize, in which case you still save
>> the overhead of creating a new process.
> "Flushing them all" is not zero-cost; it's not too hard to believe that
> it could actually be slower than forking a clean new backend.

I'm not so sure I believe that.  Is a sinval overrun slower than
forking a clean new backend?  Is DISCARD ALL slower that forking a
clean new backend?  How much white space is there between either of
those and what would be needed here?  I guess it could be slower, but
I wouldn't want to assume that without evidence.

> What's much worse, it's not zero-bug.  We've got little bitty caches
> all over the backend, including (no doubt) some caching behavior in
> third-party code that wouldn't get the word about whatever API you
> invented to deal with this.

I think this is probably the biggest issue with the whole idea, and I
agree there would be some pain involved.

Robert Haas
The Enterprise Postgres Company

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