On 13-04-23 10:04 AM, Anne Rosset wrote:
Thanks Steve.
I found this: http://www.postgresql.org/docs/current/static/release-9-2-3.html
"
Fix performance problems with autovacuum truncation in busy workloads (Jan 
Wieck)
Truncation of empty pages at the end of a table requires exclusive lock, but 
autovacuum was coded to fail (and release the table lock) when there are conflicting 
lock requests. Under load, it is easily possible that truncation would never occur, 
resulting in table bloat. Fix by performing a partial truncation, releasing the 
lock, then attempting to re-acquire the lock and continue. This fix also greatly 
reduces the average time before autovacuum releases the lock after a conflicting 
request arrives."

So that is not the fix?
No, that is the change that caused this problem. That fix addresses a slightly different set of symptoms where the truncate as part of an auto-vacuum doesn't happen because the lock gets pre-empted. An unintended/undesirable consequence of that fix was that it means if vacuum can't do the truncate stage because it can't obtain the lock in the first place then statistics don't get updated.



(Sorry to ask a second time but I really need to make sure).
Thanks,
Anne



-----Original Message-----


From: Steve Singer [mailto:st...@ssinger.info]
Sent: Tuesday, April 23, 2013 6:33 AM
To: Anne Rosset
Cc: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Performance with the new security release?

On 13-04-22 11:46 PM, Anne Rosset wrote:
Thanks Steve.
I have read that a fix has been put in release 9.2.3 for this issue. Is that 
right?
Thanks,
Anne
No this issue is present in 9.0.13, 9.1.9 and 9.2.4 (as well as 9.2.3).

There is talk about fixing this for the next set of minor releases but I 
haven't yet seen a patch.

-----Original Message-----
From: Steve Singer [mailto:st...@ssinger.info]
Sent: Monday, April 22, 2013 4:35 PM
To: Anne Rosset
Cc: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Performance with the new security release?

On 13-04-22 04:41 PM, Anne Rosset wrote:
Hi Steve,
Yes I see these messages in our log. Is there a solution to this?
Thanks,
Anne
A manual analyze of the effected tables should work and give you updated 
statistics.  If your problem is just statistics then that should help.
A manual vacuum will , unfortunately, behave like the auto-vacuum. The only way 
to get vacuum past this (until this issue is fixed) is for
vacuum to be able to get that exclusive lock.   If there are times of
the day your database is less busy you might have some luck turning off 
auto-vacuum on these tables  and doing manual vacuums during those times.




-----Original Message-----
From: Steve Singer [mailto:st...@ssinger.info]
Sent: Monday, April 22, 2013 1:26 PM
To: Anne Rosset
Cc: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Performance with the new security release?

On 13-04-22 04:15 PM, Anne Rosset wrote:
Hi Steve,
Thanks for your reply.
We are now running  9.0.13. Before it was 9.0.7.
How can I find out if we are running into this issue: "ie if
statistics are no longer being updated because analyze can't get the exclusive lock 
for truncation"?
This issue is discussed in the thread
http://www.postgresql.org/message-id/CAMkU=1xYXOJp=jLAASPdSAqab-HwhA_
t
nRhy+JUe=4=b=v3...@mail.gmail.com

If your seeing messages in your logs of the form:

automatic vacuum of table XXX.YYY cannot (re)acquire exclusive lock for truncate 
scan"

then you might be hitting this issue.


I will dig into our logs to see for the query times.
Thanks,
Anne

-----Original Message-----
From: Steve Singer [mailto:st...@ssinger.info]
Sent: Monday, April 22, 2013 12:59 PM
To: Anne Rosset
Cc: pgsql-hackers@postgresql.org
Subject: Re: [HACKERS] Performance with the new security release?

On 13-04-22 01:38 PM, Anne Rosset wrote:
Hi,
We are seeing some overall performance degradation in our application
      since we installed the security release. Other commits were
also done at the same time in the application so we don't know yet if the
      degradation has any relationship with the security release.

      While we are digging into this, I would like to know if it is possible
      that the release has some impact on performance. After reading this
      "It was created as a side effect of a refactoring effort to make
      establishing new connections to a PostgreSQL server faster, and the
      associated code more maintainable.", I am thinking it is quite possible.

      Please let me know. Thanks,
Exactly which version of PostgreSQL are you running? (we released security 
update releases for multiple PG versions).  Also which version were you running 
before?

There were some changes to analyze/vacuum in the previous set of minor releases 
that could cause performance issues in some cases (ie if statistics are no 
longer being updated because analyze can't get the
exclusive lock for truncation).   There might be other unintended
performance related changes.

Are all queries taking longer or only some?  Can you find any sort of pattern 
that might help narrow the issue?

Steve

      Anne








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