Re: Performance with the new security release? - Mailing list pgsql-hackers

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
releasethe table lock) when there are conflicting lock requests. Under load, it is easily possible that truncation
wouldnever occur, resulting in table bloat. Fix by performing a partial truncation, releasing the lock, then attempting
tore-acquire the lock and continue. This fix also greatly reduces the average time before autovacuum releases the lock
aftera 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:steve@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:steve@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
statisticsthen that should help.
 
>> A manual vacuum will , unfortunately, behave like the auto-vacuum. The only way to get vacuum past this (until this
issueis 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
manualvacuums during those times.
 
>>
>>
>>
>>
>>> -----Original Message-----
>>> From: Steve Singer [mailto:steve@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=v3KoQ@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:steve@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
issuesin 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
>>>>>
>>>>>
>>
>
>