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Re: SSI bug?

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: <drkp(at)csail(dot)mit(dot)edu>,"YAMAMOTO Takashi" <yamt(at)mwd(dot)biglobe(dot)ne(dot)jp>
Cc: <heikki(dot)linnakangas(at)enterprisedb(dot)com>, <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: SSI bug?
Date: 2011-04-07 20:34:19
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
I wrote:
> YAMAMOTO Takashi <yamt(at)mwd(dot)biglobe(dot)ne(dot)jp> wrote:
>> LOG:  could not truncate directory "pg_serial": apparent
>> wraparound
> there's some sort of cleanup bug to fix in the predicate
> locking's use of SLRU. It may be benign, but we won't really know
> until we find it.  I'm investigating.
I'm pretty sure I found it.  When the number serializable
transactions which need to be tracked gets high enough to push
things to the SLRU summarization, and then drops back down, we
haven't been truncating the head page of the active SLRU region
because if we go back into SLRU summarization that saves us from
zeroing the page again.  The problem is that if we don't go back
into SLRU summarization for a long time, we might wrap around to
where SLRU is upset that our head is chasing our tail.  This seems
like a bigger problem than we were trying to solve by not truncating
the page.
The issue is complicated a little bit by the fact that the SLRU API
has you specify the *page* for the truncation point, but silently
ignores the request unless the page is in a segment which is past a
segment in use.  So adding the number of pages per SLRU segment to
the head page position should do the right thing.  But it's all
weird enough that I felt it need a bit of commenting.
While I was there I noticed that we're doing the unnecessary
flushing (so people can glean information about the SLRU activity
from watching the disk files) right before truncating.  I switched
the truncation to come before the flushing, since flushing pages to
a file and then deleting that file didn't seem productive.
Attached find a patch which modifies one line of code, switches the
order of two lines of code, and adds comments.
I will add this to the open items for 9.1.  Thanks again to YAMAMOTO
Takashi for his rigorous testing.

Attachment: ssi-slru-truncate-fix.patch
Description: text/plain (2.3 KB)

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