linux-kernel - RE: [PATCH v2 0/3] staging: zcache: xcfmalloc support

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Date:	Thu, 15 Sep 2011 10:29:51 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Nitin Gupta <ngupta@...are.org>
Cc:	Greg KH <greg@...ah.com>, gregkh@...e.de,
	devel@...verdev.osuosl.org, cascardo@...oscopio.com,
	linux-kernel@...r.kernel.org, dave@...ux.vnet.ibm.com,
	linux-mm@...ck.org, brking@...ux.vnet.ibm.com,
	rcj@...ux.vnet.ibm.com
Subject: RE: [PATCH v2 0/3] staging: zcache: xcfmalloc support

> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
> Subject: Re: [PATCH v2 0/3] staging: zcache: xcfmalloc support
> 
> Hey Nitin,
> 
> So this is how I see things...
> 
> Right now xvmalloc is broken for zcache's application because
> of its huge fragmentation for half the valid allocation sizes
> (> PAGE_SIZE/2).

Um, I have to disagree here. It is broken for zcache for
SOME set of workloads/data, where the AVERAGE compression
is poor (> PAGE_SIZE/2).
 
> My xcfmalloc patches are _a_ solution that is ready now.  Sure,
> it doesn't so compaction yet, and it has some metadata overhead.
> So it's not "ideal" (if there is such I thing). But it does fix
> the brokenness of xvmalloc for zcache's application.

But at what cost?  As Dave Hansen pointed out, we still do
not have a comprehensive worst-case performance analysis for
xcfmalloc.  Without that (and without an analysis over a very
large set of workloads), it is difficult to characterize
one as "better" than the other.

> So I see two ways going forward:
> 
> 1) We review and integrate xcfmalloc now.  Then, when you are
> done with your allocator, we can run them side by side and see
> which is better by numbers.  If yours is better, you'll get no
> argument from me and we can replace xcfmalloc with yours.
> 
> 2) We can agree on a date (sooner rather than later) by which your
> allocator will be completed.  At that time we can compare them and
> integrate the best one by the numbers.
> 
> Which would you like to do?

Seth, I am still not clear why it is not possible to support
either allocation algorithm, selectable at runtime.  Or even
dynamically... use xvmalloc to store well-compressible pages
and xcfmalloc for poorly-compressible pages.  I understand
it might require some additional coding, perhaps even an
ugly hack or two, but it seems possible.

Dan
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