Re: General performance/load issue

Le 24 novembre 2011 17:02, Tomas Vondra <tv(at)fuzzy(dot)cz> a écrit :
> On 24 Listopad 2011, 16:39, Robert Treat wrote:
>> On Thu, Nov 24, 2011 at 9:09 AM, Tomas Vondra <tv(at)fuzzy(dot)cz> wrote:
>>> On 24 Listopad 2011, 14:51, Gaëtan Allart wrote:
>>>> Postgresql.conf :
>>>>
>>>> max_connections = 50
>>>> shared_buffers = 12G
>>>> temp_buffers = 40MB
>>>> work_mem = 128MB
>>>> maintenance_work_mem = 256MB
>>>> max_files_per_process = 8192
>>>> checkpoint_segments = 256
>>>> checkpoint_timeout = 30min
>>>> checkpoint_completion_target = 0.9
>>>
>>> Fine. Let's see the options that look suspicious.
>>>
>>
>> I think you missed some suspicious settings... I'd recommend setting
>> shared buffers to 8gb, and I'd likely reduce checkpoint segements to
>> 30 and set the checkpoint timeout back to 5 minutes. Everything about
>> the way this server is configured (including those vm settings) is
>> pushing it towards delaying the WAL/Buffer/Checkpoint as long as
>> possible, which matches with the idea of good performance initial
>> followed by a period of poor performance and heavy i/o.
>
> Yes, checkpoints were my first thought too. OTOH the OP reported that most
> of the I/O is caused by WAL writer - that's not exactly the part that does
> the work during checkpoint. Plus the WAL may not be postponed, as it's
> usually O_DIRECT and fsynced, right.
>
> You're right that the writes are postponed, but I generally see that as a
> good thing when combined with spread checkpoints. And even with those vm
> settings (about 3.2GB for background writes), I wouldn't expect this
> behaviour (because the page cache usually expires after 30 seconds). Say
> you need 100% of the shared buffers is dirty and need to be written. You
> have 27 minutes (30*0.9) to do that - that means about 8MB/s. With 30
> seconds expire there might be about 240MB before the pdflush starts to
> write the data to the SSD. And that can surely handle more than 50MB/s. So
> why the long delay? The question is what else is going on there.
>
> But all this is just guessing - I want to see the log_checkpoint message,
> iostat results etc.
>
>> On a side note, I'd guess your work_mem is probably too high. 50
>> (connections) x 128 (mb work mem) x 2 (sorts per query) = 12GB RAM,
>> which is 25% of total ram on the box. That doesn't necessarily mean
>> game over, but it seem like it wouldn't be that hard to get thrashing
>> being set up that way. YMMV.
>
> This is one of the reasons why effective_cache_size should be lower than
> 32GB, probably ...

according to 'free' output, 38GB is what is here right now.
effective_cache_size is just informative, so you can put it to 1TB
without memory issue.
And, it is OS cache+PG cache.

There is not enougth information yet to be sure on what's happening.
log_checkpoint output will help for sure.

--
Cédric Villemain +33 (0)6 20 30 22 52
http://2ndQuadrant.fr/
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