linux-kernel - Re: [PATCH] xen: core dom0 support

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Date:	Sat, 07 Mar 2009 01:06:15 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: [PATCH] xen: core dom0 support

Ingo Molnar wrote:
> Have i missed a mail of yours perhaps? I dont have any track of 
> you having posted mmap-perf perfcounters results. I grepped my 
> mbox and the last mail i saw from you containing the string 
> "mmap-perf" is from January 20, and it only includes my numbers.

Yes, I think you must have missed a mail. I've attached it for 
reference, along with a more complete set of measurements I made 
regarding the series of patches applied (series ending at 
1f4f931501e9270c156d05ee76b7b872de486304) to improve pvops performance.

My results showed a dramatic drop in cache references (from about 300% 
pvop vs non-pvop, down to 125% with the full set of patches applied), 
but it didn't seem to make much of an effect on the overall wallclock 
time. I'm a bit sceptical of the numbers here because, while each run's 
passes are fairly consistent, booting and remeasuring seemed to cause 
larger variations than we're looking at. It would be easy to handwave it 
away with "cache effects", but its not very satisfying.

I also didn't find the measurements very convincing because the number 
of CPU cycles and instructions executed count is effectively unchanged 
(ie, the baseline non-pvops vs original pvops apparently execute exactly 
the same number of instructions, but we know that there's a lot more 
going on), and with no change as each added patch definitely removes 
some amount of pvops overhead in terms of instructions in the 
instruction stream. Is it just measuring usermode stats? I ran it as 
root, with the command line you suggested ("./perfstat -e 
-5,-4,-3,0,1,2,3 ./mmap-perf 1"). Cache misses wandered up and down in a 
fairly non-intuitive way as well.

I'll do a rerun comparing current tip.git pvops vs non-pvops to see if I 
can get some better results.

J

Download attachment "pvops-mmap-measurements.ods" of type "application/vnd.oasis.opendocument.spreadsheet" (20038 bytes)

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