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

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
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