Re: drop duplicate buffers in OS
| От | Jeff Janes |
|---|---|
| Тема | Re: drop duplicate buffers in OS |
| Дата | |
| Msg-id | CAMkU=1xoX7Dxu6V433gyrfgdyLEjVLev7L8xJ7w3MkJUay4FOw@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: drop duplicate buffers in OS (Robert Haas <robertmhaas@gmail.com>) |
| Ответы |
Re: drop duplicate buffers in OS
|
| Список | pgsql-hackers |
On Wed, Jan 15, 2014 at 10:34 AM, Robert Haas <robertmhaas@gmail.com> wrote:
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.
Cheers,
Jeff
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