linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Date:	Tue, 20 Jan 2009 20:37:03 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	jeremy@...source.com, chrisw@...s-sol.org, zach@...are.com,
	rusty@...tcorp.com.au, Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT


* Ingo Molnar <mingo@...e.hu> wrote:

> -----------------------------------------------
> | Performance counter stats for './mmap-perf' |
> -----------------------------------------------
> |                |
> |  x86-defconfig |   PARAVIRT=y         
> |------------------------------------------------------------------
> |
> |    1311.554526 |  1360.624932  task clock ticks (msecs)    +3.74%
> |                |
> |              1 |            1  CPU migrations
> |             91 |           79  context switches
> |          55945 |        55943  pagefaults
> |    ............................................
> |     3781392474 |   3918777174  CPU cycles                  +3.63%
> |     1957153827 |   2161280486  instructions               +10.43%
> |       50234816 |     51303520  cache references            +2.12%
> |        5428258 |      5583728  cache misses                +2.86%
> |                |
> |    1314.782469 |  1363.694447  time elapsed (msecs)        +3.72%
> |                |
> -----------------------------------
> 
> The most surprising element is that in the paravirt_ops case we run 204 
> million more instructions - out of the ~2000 million instructions total.

So because this test does exactly 1 million MM syscalls, the average is 
easy to calculate:

The native kernel's average MM syscall cost is 1957 instructions - with 
CONFIG_PARAVIRT=y that increases by +10.43% to 2161 instructions. There's 
over 200 extra instructions executed per MM syscall that we only do due to 
CONFIG_PARAVIRT=y.

	Ingo
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