| From: | Andres Freund <andres(at)2ndquadrant(dot)com> |
|---|---|
| To: | James Bottomley <James(dot)Bottomley(at)HansenPartnership(dot)com> |
| Cc: | Jim Nasby <jim(at)nasby(dot)net>, Claudio Freire <klaussfreire(at)gmail(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Kevin Grittner <kgrittn(at)ymail(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>, Joshua Drake <jd(at)commandprompt(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, Mel Gorman <mgorman(at)suse(dot)de>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, "lsf-pc(at)lists(dot)linux-foundation(dot)org" <lsf-pc(at)lists(dot)linux-foundation(dot)org>, Magnus Hagander <magnus(at)hagander(dot)net> |
| Subject: | Re: [Lsf-pc] Linux kernel impact on PostgreSQL performance |
| Date: | 2014-01-13 21:12:31 |
| Message-ID: | 20140113211231.GA5838@awork2.anarazel.de |
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| Lists: | pgsql-hackers |
On 2014-01-13 12:34:35 -0800, James Bottomley wrote:
> On Mon, 2014-01-13 at 14:32 -0600, Jim Nasby wrote:
> > Well, if we were to collaborate with the kernel community on this then
> > presumably we can do better than that for eviction... even to the
> > extent of "here's some data from this range in this file. It's (clean|
> > dirty). Put it in your cache. Just trust me on this."
>
> This should be the madvise() interface (with MADV_WILLNEED and
> MADV_DONTNEED) is there something in that interface that is
> insufficient?
For one, postgres doesn't use mmap for files (and can't without major
new interfaces). Frequently mmap()/madvise()/munmap()ing 8kb chunks has
horrible consequences for performance/scalability - very quickly you
contend on locks in the kernel.
Also, that will mark that page dirty, which isn't what we want in this
case. One major usecase is transplanting a page comming from postgres'
buffers into the kernel's buffercache because the latter has a much
better chance of properly allocating system resources across independent
applications running.
Oh, and the kernel's page-cache management while far from perfect,
actually scales much better than postgres'.
Greetings,
Andres Freund
--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
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