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

On Tue, Jan 14, 2014 at 2:12 PM, Robert Haas <robertmhaas@gmail.com> wrote:
>
> In terms of avoiding double-buffering, here's my thought after reading
> what's been written so far.  Suppose we read a page into our buffer
> pool.  Until the page is clean, it would be ideal for the mapping to
> be shared between the buffer cache and our pool, sort of like
> copy-on-write.  That way, if we decide to evict the page, it will
> still be in the OS cache if we end up needing it again (remember, the
> OS cache is typically much larger than our buffer pool).  But if the
> page is dirtied, then instead of copying it, just have the buffer pool
> forget about it, because at that point we know we're going to write
> the page back out anyway before evicting it.
>
> This would be pretty similar to copy-on-write, except without the
> copying.  It would just be forget-from-the-buffer-pool-on-write.


But... either copy-on-write or forget-on-write needs a page fault, and
thus a page mapping.

Is a page fault more expensive than copying 8k?

(I really don't know).