On Wed, Jan 15, 2014 at 1:53 AM, KONDO Mitsumasa <kondo.mitsumasa@lab.ntt.co.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...
Have you had any luck with it? I have reservations about this approach. Among other reasons, if the buffer is truly nailed in shared_buffers for the long term, the kernel won't see any activity on it and will be able to evict it fairly efficiently on its own.
So I'm reluctant to do a detailed review if the author cannot demonstrate a performance improvement. I'm going to mark it waiting-on-author for that reason.
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.
I'm surprised that that is common. The only cases I've seen that was either when the database exactly fits in shared_buffers, or when the database is mostly appended, and the appends are done with inserts in a loop rather than COPY.