Обсуждение: Context switching

Reimer
carlos.reimer@opendb.com.br
OpenDB Serviços e Treinamentos PostgreSQL e DB2
Fone: 47 3327-0878 Cel: 47 9602-0151
www.opendb.com.br

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.

 

CPU and memory usage are ok.

 

What can produce this context switching storms?

 

It is a box with 12GB RAM and 4 processors running RedHat Enterprise Linux AS.

 

Thank you in advance!

Reimer
carlos.reimer@opendb.com.br
OpenDB Serviços e Treinamentos PostgreSQL e DB2
Fone: 47 3327-0878 Cel: 47 9602-0151
www.opendb.com.br

 

Carlos,

> 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.

Context Switching is a symptom rather than a cause.   What's most likely
happening is that you have a combined heavy-CPU and heavy-IO workload, so you
have bursts of CPU activity stalled by iowaits.

Can you check the rate of iowaits during the "storm" periods?

Also, is this Xeon?   And are you saying that you *didn't* have this issue
under 7.4?

--
Josh Berkus
PostgreSQL @ Sun
San Francisco

Hi,

I have not seen the iowaits numbers and yes, it is a xeon, four processors.

With the 7.4 version the problem did not exist but I think Tom Lane cleared why this behavior with 8.0 and 8.1 versions
inhttp://archives.postgresql.org/pgsql-performance/2006-11/msg00050.php 

Reimer


> -----Mensagem original-----
> De: Josh Berkus [mailto:josh@agliodbs.com]
> Enviada em: terça-feira, 7 de novembro de 2006 15:42
> Para: pgsql-performance@postgresql.org; carlos.reimer@opendb.com.br
> Assunto: Re: [PERFORM] Context switching
>
>
> Carlos,
>
> > 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.
>
> Context Switching is a symptom rather than a cause.   What's most likely
> happening is that you have a combined heavy-CPU and heavy-IO
> workload, so you
> have bursts of CPU activity stalled by iowaits.
>
> Can you check the rate of iowaits during the "storm" periods?
>
> Also, is this Xeon?   And are you saying that you *didn't* have
> this issue
> under 7.4?
>
> --
> Josh Berkus
> PostgreSQL @ Sun
> San Francisco
>
>