[pgpool-general: 613] Re: load balancing seems to be bottlenecked by performance of master

[Lonni J Friedman]

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.

thanks