Re: CLOG contention, part 2

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 <
TransactionIdToPage(RecentXmin))
ClogCtl->shared->RecentXminPageno =
TransactionIdToPage(RecentXmin);

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);

Cheers,

Jeff