Re: Context switch storm

[Cosimo Streppone <cosimo(at)streppone(dot)it>]

Richard Huxton wrote:

> creimer(at)brturbo(dot)com(dot)br wrote:
> > Hi,
> >
> > We've migrated one of our servers from pg 7.4 to 8.1 and from times
> > to times (4 hours) the server start doing a lot of context switching
> > and all transactions become very slow.
> >
> > 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 ?
I seem to have the same exact behaviour for an OLTP-loaded 8.0.1 server
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.

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.

When the load increases, *random* database queries show this "slowness",
even if they are perfectly planned and indexed.

Is there anything we can do?

--
Cosimo