Re: Context switch storm

Cosimo Streppone wrote:
> Richard Huxton wrote:
>
>> creimer@brturbo.com.br wrote:
>>>
>>> The average context switching for this server as vmstat shows is 1
>>> but when the problem occurs it goes to 250000.
>>
>> You'll tend to see it when you have multiple clients and most queries
>> can use RAM rather than disk I/O. My understanding of what happens is
>> that PG requests data from RAM - it's not in cache so the process gets
>> suspended to wait. The next process does the same, with the same
>> result.   You end up with lots of processes all fighting over what
>> data is in the cache and no-one gets much work done.
>
> Does this happen also with 8.0, or is specific to 8.1 ?

All versions suffer to a degree - they just push the old Xeon in the
wrong way. However, more recent versions *should* be better than older
versions. I believe some work was put in to prevent contention on
various locks which should reduce context-switching across the board.

> I seem to have the same exact behaviour for an OLTP-loaded 8.0.1 server

upgrade from 8.0.1 - the most recent is 8.0.9 iirc

> when I raise `shared_buffers' from 8192 to 40000.
> I would expect an increase in tps/concurrent clients, but I see an average
> performance below a certain threshold of users, and when concurrent users
> get above that level, performance starts to drop, no matter what I do.

Are you seeing a jump in context-switching in top? You'll know when you
do - it's a *large* jump. That's the key diagnosis. Otherwise it might
simply be your configuration settings aren't ideal for that workload.

> Server logs and io/vm statistics seem to indicate that there is little
> or no disk activity but machine loads increases to 7.0/8.0.
> After some minutes, the problem goes away, and performance returns
> to acceptable levels.

That sounds like it. Query time increases across the board as all the
clients fail to get any data back.

> When the load increases, *random* database queries show this "slowness",
> even if they are perfectly planned and indexed.
>
> Is there anything we can do?

Well, the client I saw it with just bought a dual-opteron server and
used their quad-Xeon for something else. However, I do remember that 8.1
seemed better than 7.4 before they switched. Part of that might just
have been better query-planning and other efficiences though.

--
   Richard Huxton
   Archonet Ltd