Re: [Lsf-pc] Linux kernel impact on PostgreSQL performance

On Wed, Jan 15, 2014 at 1:35 PM, Stephen Frost <sfrost(at)snowman(dot)net> wrote:
>> And there's a nice bingo. Had forgotten about KSM. KSM could help lots.
>>
>> I could try to see of madvising shared_buffers as mergeable helps. But
>> this should be an automatic case of KSM - ie, when reading into a
>> page-aligned address, the kernel should summarily apply KSM-style
>> sharing without hinting. The current madvise interface puts the burden
>> of figuring out what duplicates what on the kernel, but postgres
>> already knows.
>
> I'm certainly curious as to if KSM could help here, but on Ubuntu 12.04
> with 3.5.0-23-generic, it's not doing anything with just PG running.
> The page here: http://www.linux-kvm.org/page/KSM seems to indicate why:
>
> ----
> KSM is a memory-saving de-duplication feature, that merges anonymous
> (private) pages (not pagecache ones).
> ----
>
> Looks like it won't merge between pagecache and private/application
> memory? Or is it just that we're not madvise()'ing the shared buffers
> region? I'd be happy to test doing that, if there's a chance it'll
> actually work..

Yes, it's onlyl *intended* for merging private memory.

But, still, the implementation is very similar to what postgres needs:
sharing a physical page for two distinct logical pages, efficiently,
with efficient copy-on-write.

So it'd be just a matter of removing that limitation regarding page
cache and shared pages.

If you asked me, I'd implement it as copy-on-write on the page cache
(not the user page). That ought to be low-overhead.