On Wednesday, December 12, 2012 5:23 AM Greg Smith wrote:
> On 11/23/12 5:57 AM, Amit kapila wrote:
> > Let us try to see by example:
> > Total RAM - 22G
> > Database size - 16G
> >...
> > Case -2 (Shared Buffers - 10G)
> > a. Load all the files in OS buffers. In best case OS buffers can
> contain10-12G data as OS has 12G of memory available.
> > b. Try to load all in Shared buffers. Last 10G will be there in shared
> buffers.
> > c. Now as there is no direct correlation of data between Shared
> Buffers and OS buffers, so whenever PG has to access any data
> > which is not there in Shared Buffers, good chances are there that
> it can lead to IO.
>
> I don't think either of these examples are very representative of
> real-world behavior. The idea of "load all the files in OS buffers"
> assumes someone has used a utility like pg_prewarm or pgfincore. It's
> not something that happens in normal use. Being able to re-populate all
> of RAM using those utilities isn't realistic anyway. Anyone who tries
> to load more than (memory - shared_buffers) that way is likely to be
> disappointed by the result.
True, I also think nobody will directly try to do it in this way, but such
similar situations can arise after long run.
Something like if we assume most used pages fall under the range of RAM.
> Similarly, the problem you're describing here has been described as the
> "double buffering" one for a while now. The old suggestion that
> shared_buffers not be set about 25% of RAM comes from this sort of
> concern. If triggering a problem requires doing that, essentially
> misconfiguring the server, it's hard to get too excited about it.
>
> Anyway, none of that impacts on me mixing testing for this into what I'm
> working on. The way most pgbench tests happen, it's hard to *not* have
> the important data in cache. Once you run the init step, you have to
> either reboot or drop the OS cache to get those pages out of RAM. That
> means the sort of cached setup you're using pg_prewarm to
> simulate--things are in the OS cache, but not the PostgreSQL one--is one
> that anyone running an init/test pair will often create. You don't need
> pg_prewarm to do it.
The way, I have ran the tests is to just try to simulate scenario's where
invalidating buffers
by bgwriter/checkpoint can have advantage.
With Regards,
Amit Kapila.
