linux-kernel - Re: [GIT PULL] mm: frontswap (for 3.2 window)

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Message-ID: <4EB1AD53.2000600@redhat.com>
Date:	Wed, 02 Nov 2011 16:51:31 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Andrea Arcangeli <aarcange@...hat.com>,
	Pekka Enberg <penberg@...nel.org>,
	Cyclonus J <cyclonusj@...il.com>,
	Sasha Levin <levinsasha928@...il.com>,
	Christoph Hellwig <hch@...radead.org>,
	David Rientjes <rientjes@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-mm@...ck.org, LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>, ngupta@...are.org,
	Chris Mason <chris.mason@...cle.com>, JBeulich@...ell.com,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Jonathan Corbet <corbet@....net>
Subject: Re: [GIT PULL] mm: frontswap (for 3.2 window)

On 10/31/2011 07:36 PM, Dan Magenheimer wrote:
>> From: Andrea Arcangeli [mailto:aarcange@...hat.com]

>>> real work to do instead and (2) that vmexit/vmenter is horribly
>>
>> Sure the CPU has another 1000 VM to schedule. This is like saying
>> virtio-blk isn't needed on desktop virt becauase the desktop isn't
>> doing much I/O. Absurd argument, there are another 1000 desktops doing
>> I/O at the same time of course.
>
> But this is truly different, I think at least for the most common
> cases, because the guest is essentially out of physical memory if it
> is swapping.  And the vmexit/vmenter (I assume, I don't really
> know KVM) gives the KVM scheduler the opportunity to schedule
> another of those 1000 VMs if it wishes.

I believe the problem Andrea is trying to point out here is
that the proposed API cannot handle a batch of pages to be
pushed into frontswap/cleancache at one time.

Even if the current back-end implementations are synchronous
and can only do one page at a time, I believe it would still
be a good idea to have the API able to handle a vector with
a bunch of pages all at once.

That way we can optimize the back-ends as required, at some
later point in time.

If enough people start using tmem, such bottlenecks will show
up at some point :)

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