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

Robert Haas <robertmhaas@gmail.com> writes:
> On Tue, Jan 14, 2014 at 11:57 AM, James Bottomley
> <James.Bottomley@hansenpartnership.com> wrote:
>> No, I do ... you mean the order of write out, if we have to do it, is
>> important.  In the rest of the kernel, we do this with barriers which
>> causes ordered grouping of I/O chunks.  If we could force a similar
>> ordering in the writeout code, is that enough?

> Probably not.  There are a whole raft of problems here.  For that to
> be any of any use, we'd have to move to mmap()ing each buffer instead
> of read()ing them in, and apparently mmap() doesn't scale well to
> millions of mappings.

We would presumably mmap whole files, not individual pages (at least
on 64-bit machines; else address space size is going to be a problem).
However, without a fix for the critical-section/atomic-update problem,
the idea's still going nowhere.

> This would be pretty similar to copy-on-write, except without the
> copying.  It would just be forget-from-the-buffer-pool-on-write.

That might possibly work.
        regards, tom lane