linux-kernel - Re: userspace pagecache management tool

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Message-ID: <45EA1F7B.9060107@redhat.com>
Date:	Sat, 03 Mar 2007 20:23:07 -0500
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: userspace pagecache management tool

Andrew Morton wrote:
> On Sat, 03 Mar 2007 19:01:01 -0500 Rik van Riel <riel@...hat.com> wrote:

>> The use-once policy we have in the kernel should work
>> perfectly fine for backups.  All we need to do is
>> actually honor the accessed bit on active page cache
>> pages, instead of flushing them onto the inactive
>> list.
>>
>> What am I overlooking?
> 
> That'll improve backups but will break other things.
> 
> To do this effectively we'd need to change the policy so that new pagecache
> allocations cause no scanning of used-twice pages at all.  So that even
> after many gigs of backing up, the working set is still there.
> 
> Problem is, (for example) what about the person who has 80% of memory in
> used-twice state and who then reads a file or files which are 20% or more of
> the size of memory, two or more times.  It'll be 100% cache misses, every time.
> This will happen quite a lot.  IOW, once those pages are in used-twice state,
> how does further pagecache activity ever get them _out_ of that state?  Only
> by joining the used-twice page set, and that can't happen if the used-once-so-far
> pages got reclaimed.
> 
> Doing a refault thing would help a bit, but stops working at a certain point.

At what point does it stop working?

I am not asking this to be difficult, I just want to get Linux
a VM that does not need to be kludged up every time a distro
ships it to its customers.

I believe one starting point would be a concept that people
cannot shoot holes in any more.  That is no guarantee, but
as long as the concept has known holes coding it up is likely
to be a waste of time since the code will need kludges to
deal with the problems later on and we'd be back to square
one.

-- 
Politics is the struggle between those who want to make their country
the best in the world, and those who believe it already is.  Each group
calls the other unpatriotic.
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