[pgpool-general: 615] Re: load balancing seems to be bottlenecked by performance of master

[Tatsuo Ishii]

> On Thu, Jun 7, 2012 at 11:01 AM, Lonni J Friedman <netllama at gmail.com> wrote:
>> On Tue, Jun 5, 2012 at 6:06 PM, Lonni J Friedman <netllama at gmail.com> wrote:
>>> On Tue, Jun 5, 2012 at 5:26 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>>>>> On Tue, Jun 5, 2012 at 4:13 PM, Tatsuo Ishii <ishii at sraoss.co.jp> wrote:
>>>>>>> On Mon, Jun 4, 2012 at 10:21 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>>>>>>>>> On Mon, Jun 4, 2012 at 3:40 PM, Tatsuo Ishii <ishii at postgresql.org> wrote:
>>>>>>>>>>> You're correct, I only needed to perform a reload.  I kept this change
>>>>>>>>>>> enabled for all of 36 seconds, and in that time there were 1597 times
>>>>>>>>>>> that the following query was logged originating from the pgpool server
>>>>>>>>>>> IP address:
>>>>>>>>>>> SELECT count(*) FROM pg_catalog.pg_class AS c WHERE c.oid ...
>>>>>>>>>>>
>>>>>>>>>>> 3204 times that the following query was logged originating from the
>>>>>>>>>>> pgpool server IP address:
>>>>>>>>>>> SELECT count(*) FROM pg_class AS c, pg_namespace AS n WHERE c.relname ...
>>>>>>>>>>>
>>>>>>>>>>> Those numbers seem excessive to me, but perhaps this is
>>>>>>>>>>> expected/normal?  It definitely seems like it would explain why perf
>>>>>>>>>>> always degrades whenever the master is very busy, if that volume of
>>>>>>>>>>> queries needs to be sustained.
>>>>>>>>>>
>>>>>>>>>> That depends on your use case. If client's query involves many tables,
>>>>>>>>>> it requires many catalog lookups. Also the catalog cache of pgpool's
>>>>>>>>>> life time is same as pgpool child process lifetime.  If you would show
>>>>>>>>>> me the complete log, I could make more precise analysis.
>>>>>>>>>
>>>>>>>>> Sure, attached as pg.log.gz.
>>>>>>>>
>>>>>>>> Analyzing the log I noticed several things:
>>>>>>>>
>>>>>>>> 1) I see many sessions which frequently connects/disconnects(over 1800
>>>>>>>>   in 36 seconds). That is, each session only lasts 20 ms in average.
>>>>>>>>
>>>>>>>> 2) A process issues exactly same query more than once. This suggests
>>>>>>>>   that relcache (catalog cache) size might be too small for your
>>>>>>>>   environment. The size of the cache is vary from cache to cache, but
>>>>>>>>   typical size is 128. So if you access more than 128 tables via
>>>>>>>>   pgpool, the cache replacement will happen. How many tables do you
>>>>>>>>   have?
>>>>>>>
>>>>>>> hundreds.  I don't see any option for setting the relcache size.  How
>>>>>>> do I make this change?
>>>>>>
>>>>>> Currently the only way is changing the source. pgpool-II 3.2 will has
>>>>>> new directive(relcache_size) to control this.
>>>>>
>>>>> When do you anticipate that 3.2 will be released?
>>>>
>>>> Officially not decided yet. I personally expect 3.2 is going to be out
>>>> by the end of this month.
>>>>
>>>>> Where in the source of 3.1.x would I make this change, and what would
>>>>> you suggest I change it to?
>>>>
>>>> grep "pool_create_relcache(" *.c will show places where relcache is
>>>> created. pool_relcache's first argument is the number of cache
>>>> entries, which you would want to increase.
>>
>> I don't have a good understanding of what the number represents, or
>> how much I'd ideally need.   what would you suggest increasing the
>> value to, or how can I determine a good value?
> 
> I can't tell if the lack of a reply is because you don't have time to
> answer, or for some other reason.  I didn't think my questions were
> unreasonable.  I'd really appreciate some guidance on how to address
> this, as its a serious issue in my environment.

You should increase them until cache replacement does not
happen. Problem is, how to know cache replacement? (Pgpool-II 3.2 will
emit log in this case)

So my only suggestion is, increase to number of tables you have in
your database.
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp