Re: Linux kernel impact on PostgreSQL performance

On Mon, Jan 13, 2014 at 5:23 PM, Jim Nasby <jim(at)nasby(dot)net> wrote:
> On 1/13/14, 2:19 PM, Claudio Freire wrote:
>>
>> On Mon, Jan 13, 2014 at 5:15 PM, Robert Haas <robertmhaas(at)gmail(dot)com>
>> wrote:
>>>
>>> On a related note, there's also the problem of double-buffering. When
>>> we read a page into shared_buffers, we leave a copy behind in the OS
>>> buffers, and similarly on write-out. It's very unclear what to do
>>> about this, since the kernel and PostgreSQL don't have intimate
>>> knowledge of what each other are doing, but it would be nice to solve
>>> somehow.
>>
>>
>>
>> There you have a much harder algorithmic problem.
>>
>> You can basically control duplication with fadvise and WONTNEED. The
>> problem here is not the kernel and whether or not it allows postgres
>> to be smart about it. The problem is... what kind of smarts
>> (algorithm) to use.
>
>
> Isn't this a fairly simple matter of when we read a page into shared buffers
> tell the kernel do forget that page? And a corollary to that for when we
> dump a page out of shared_buffers (here kernel, please put this back into
> your cache).

That's my point. In terms of kernel-postgres interaction, it's fairly simple.

What's not so simple, is figuring out what policy to use. Remember,
you cannot tell the kernel to put some page in its page cache without
reading it or writing it. So, once you make the kernel forget a page,
evicting it from shared buffers becomes quite expensive.