Re: drop duplicate buffers in OS

On Wed, Jan 15, 2014 at 1:53 AM, KONDO Mitsumasa
<kondo(dot)mitsumasa(at)lab(dot)ntt(dot)co(dot)jp> wrote:
> I create patch that can drop duplicate buffers in OS using usage_count
> alogorithm. I have developed this patch since last summer. This feature seems to
> be discussed in hot topic, so I submit it more faster than my schedule.
>
> When usage_count is high in shared_buffers, they are hard to drop from
> shared_buffers. However, these buffers wasn't required in file cache. Because
> they aren't accessed by postgres(postgres access to shared_buffers).
> So I create algorithm that dropping file cache which is high usage_count in
> shared_buffers and is clean state in OS. If file cache are clean state in OS, and
> executing posix_fadvice DONTNEED, it can only free in file cache without writing
> physical disk. This algorithm will solve double-buffered situation problem and
> can use memory more efficiently.
>
> I am testing DBT-2 benchmark now...

The thing about this is that our usage counts for shared_buffers don't
really work right now; it's common for everything, or nearly
everything, to have a usage count of 5. So I'm reluctant to rely on
that for much of anything.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company