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Re: CLOG contention, part 2

From: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
To: Simon Riggs <simon(at)2ndquadrant(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: 2012-01-29 22:11:35
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Lists: pgsql-hackers
On Sun, Jan 29, 2012 at 1:41 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> On Sun, Jan 29, 2012 at 12:18 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> 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 for looking at the patch.
>> The patch works fine. RecentXminPageno does move forwards as it is
>> supposed to and there are no uints anywhere in that calculation.
> Maybe it is system dependent.  Or, are you running this patch on top
> of some other uncommitted patch (other than the pgbench one)?
> RecentXmin is a TransactionID, which is a uint32.
> I think the TransactionIdToPage macro preserves that.
> If I cast to a int, then I see advancement:
> if (ClogCtl->shared->RecentXminPageno < (int) TransactionIdToPage(RecentXmin))

And to clarify, if I don't do the cast, I don't see advancement, using
this code:

                elog(LOG, "JJJ RecentXminPageno %d, %d",
ClogCtl->shared->RecentXminPageno , TransactionIdToPage(RecentXmin));
                if (ClogCtl->shared->RecentXminPageno <
                        ClogCtl->shared->RecentXminPageno =

Then using your pgbench -I -s 100 -c 8 -j8, I get tons of log entries like:

LOG:  JJJ RecentXminPageno -1, 149
STATEMENT:  INSERT INTO pgbench_accounts (aid, bid, abalance) VALUES
(nextval('pgbench_accounts_load_seq'), 1 + (lastval()/(100000)), 0);



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Subject: Re: CLOG contention, part 2
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