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Re: CLOG contention, part 2

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: CLOG contention, part 2
Date: 2012-01-28 13:52:52
Message-ID: CA+U5nMLv4ddV3JoK4hK7zdUb50WMiLJxZEkHmGskctnY-FMXfw@mail.gmail.com (view raw or flat)
Thread:
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On Fri, Jan 27, 2012 at 10:05 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> On Sat, Jan 21, 2012 at 7:31 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>>
>> Yes, it was. Sorry about that. New version attached, retesting while
>> you read this.
>
> In my hands I could never get this patch to do anything.  The new
> cache was never used.
>
> I think that that was because RecentXminPageno never budged from -1.
>
> I think that that, in turn, is because the comparison below can never
> return true, because the comparison is casting both sides to uint, and
> -1 cast to uint is very large
>
>        /* When we commit advance ClogCtl's shared RecentXminPageno if needed */
>        if (ClogCtl->shared->RecentXminPageno < TransactionIdToPage(RecentXmin))
>                 ClogCtl->shared->RecentXminPageno =
> TransactionIdToPage(RecentXmin);

Thanks, will look again.

> Also, I think the general approach is wrong.  The only reason to have
> these pages in shared memory is that we can control access to them to
> prevent write/write and read/write corruption.  Since these pages are
> never written, they don't need to be in shared memory.   Just read
> each page into backend-local memory as it is needed, either
> palloc/pfree each time or using a single reserved block for the
> lifetime of the session.  Let the kernel worry about caching them so
> that the above mentioned reads are cheap.

Will think on that.

-- 
 Simon Riggs                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services

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