[pgpool-general: 606] Re: load balancing seems to be bottlenecked by performance of master

Lonni J Friedman netllama at gmail.com
Fri Jun 8 03:01:37 JST 2012


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?

thanks


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