[pgpool-general: 600] Re: load balancing seems to be bottlenecked by performance of master

[Tatsuo Ishii]

> 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.
--
Tatsuo Ishii
SRA OSS, Inc. Japan
English: http://www.sraoss.co.jp/index_en.php
Japanese: http://www.sraoss.co.jp