RE: [PATCH v2] zsmalloc: Add Kconfig for enabling PTE method

[Dan Magenheimer <dan.magenheimer@oracle.com>]

> From: Ric Mason [mailto:ric.masonn@gmail.com]
> Sent: Saturday, February 16, 2013 11:19 PM
> To: Minchan Kim
> Cc: Greg Kroah-Hartman; linux-mm@kvack.org; linux-kernel@vger.kernel.org; Andrew Morton; Seth
> Jennings; Nitin Gupta; Dan Magenheimer; Konrad Rzeszutek Wilk
> Subject: Re: [PATCH v2] zsmalloc: Add Kconfig for enabling PTE method
> 
> On 02/06/2013 10:17 AM, Minchan Kim wrote:
> > Zsmalloc has two methods 1) copy-based and 2) pte-based to access
> > allocations that span two pages. You can see history why we supported
> > two approach from [1].
> >
> > In summary, copy-based method is 3 times fater in x86 while pte-based
> > is 6 times faster in ARM.
> 
> Why in some arches copy-based method is better and in the other arches
> pte-based is better? What's the root reason?

Minchan, if you post another version, I think these precise numbers
(of "times faster") should be removed.  The speed is very data
dependent, because the copy-based method is copying a zpage which
may vary widely in size from ~100 bytes to nearly PAGE_SIZE bytes,
a factor of 40x or more.

Please at least say "up to 3 times" or "approximately 3x faster for
an average compressed page".

Ric, the copy-based method does an extra copy of N bytes (where
N is the compressed size of a page).  The pte-based method requires
extra TLB actions.  The relative speed of TLB operations vs
copying is very architecture-dependent.  It is also probably
dependent on the specific implementation of the architecture
(i.e x86 sandybridge is likely very different than x86
nehalem) and, as noted above, dependent on N which is
unpredictable.

So it makes sense to have both choices, but it's not at all clear
how to select which one to use!

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