[pgpool-general: 601] Re: load balancing seems to be bottlenecked by performance of master

[Lonni J Friedman]

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?