Re: : bg_writer overloaded ?

On Wed, Nov 9, 2011 at 8:16 PM, Scott Marlowe <scott(dot)marlowe(at)gmail(dot)com>wrote:

> On Wed, Nov 9, 2011 at 2:25 AM, Venkat Balaji <venkat(dot)balaji(at)verse(dot)in>
> wrote:
> > Hello Everyone,
> > I could see the following in the production server (result of the "top" M
> > command) -
> > PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+
> > COMMAND
> > 25265 postgres 15 0 3329m 2.5g 1.9g S 0.0 4.0
> > 542:47.83 postgres: writer process
> > The "writer process" refers to bg_writer ? and we have shared_buffers
> set to
> > 1920 MB (around 1.9 GB).
>
> So it is using 2.5G of mem of which 1.9G is shared memory (i.e. shared
> buffers) so the actual amount of RAM it's using is ~600Megs.
>
> I see no problem.
>

Is this not the indication that the shared_buffers is undersized ?

>
> > In an other similar situation, we have "postgres writer process" using
> up 7
> > - 8 GB memory constantly.
>
> I doubt it. Sounds more like you're misreading the output of top.
>

Below is the output directly taken from our production box. As per you, the
writer process is taking up 1.9g from
shared_buffers and the remaining (around 5.6 GB) from RAM.

Mem: 65980808k total, 65620700k used, 360108k free, 210792k buffers
Swap: 1052248k total, 321144k used, 731104k free, 51721468k cached

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
10306 postgres 15 0 15.7g 7.5g 1.9g S 1.9 12.0 1037:05 postgres:
writer process

Is this not a problem ?

> > pg_tune is suggesting to increase the shared_buffers to 8 GB.
>
> Reasonable.
>
> > If the shared_buffer is not enough, Postgres uses OS cache ?
>
> Not really how things work. The OS uses all spare memory as cache.
> PostgreSQL uses shared_buffers as a cache. The OS is much more
> efficient about caching in dozens of gigabytes than pgsql is.

What if the shared_buffers is not enough to cache the data being read from
the database ?

Thanks for your help !

Regards,
VB