linux-kernel - Re: [patch 04/17] Add pagetable accessors to pack and unpack pagetable entries

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Date:	Wed, 4 Apr 2007 11:57:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Andi Kleen <ak@...e.de>, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [patch 04/17] Add pagetable accessors to pack and unpack pagetable entries


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> > What do the benchmarks say with CONFIG_PARAVIRT on native hardware 
> > compared to !CONFIG_PARAVIRT. e.g. does lmbench suffer?
> 
> Barely.  There's a slight hit for not using patching, and patching is 
> almost identical to native performance.  The most noticeable 
> difference is in the null syscall microbenchmark, but once you get to 
> complex things the difference is in the noise.

i'd not call this 'barely' or 'noise'! Look:

> Processor, Processes - times in microseconds - smaller is better
> ------------------------------------------------------------------------------
> Host                 OS  Mhz null null      open slct sig  sig  fork exec sh  
>                              call  I/O stat clos TCP  inst hndl proc proc proc
> --------- ------------- ---- ---- ---- ---- ---- ---- ---- ---- ---- ---- ----
> non-paravirt
> ezr       Linux 2.6.21- 1000 0.25 0.52 31.6 34.7 10.3 1.03 5.31 726. 1565 4520
> ezr       Linux 2.6.21- 1000 0.25 0.52 31.8 34.7 12.6 1.03 5.41 725. 1564 4585
> ezr       Linux 2.6.21- 1000 0.25 0.55 31.7 34.5 11.8 1.02 5.47 720. 1595 4518
> 
> paravirt, no patching
> ezr       Linux 2.6.21- 1000 0.28 0.55 31.3 34.3 10.0 1.05 5.56 747. 1621 4675
> ezr       Linux 2.6.21- 1000 0.28 0.56 31.5 34.3 12.9 1.05 5.66 755. 1629 4684
> ezr       Linux 2.6.21- 1000 0.28 0.55 31.8 34.5 12.5 1.05 5.45 747. 1622 4695

the main metric we are interested in is the overhead for people who just 
want to run the non-patched native kernel that has CONFIG_PARAVIRT 
enabled (99%+ of the users at the moment), so the delta is:

 null:              +12.0%
 null IO:            +7.5%
 stat:        within noise
 open/close:  within noise
 TCP:                ~5.0%
 signal install:      2.0%
 signal handle:       4.7%
 fork:                2.7%
 exec:                3.6%
 shell:               3.6%

this is not 'barely measurable' but 'BLOODY LARGE' overhead. Really. I 
mean for something as complex as exec we still are 'a couple of percent' 
slower? Linux has literally _dozens_ of important but inactive features 
in every critical fastpath and in every critical system call, and if 
each took 'just a few percent', we'd be a few _times_ slower as an end 
result, for features that are not even used! The answer to any such 
overhead is: "get your act together and stop burdening those who dont 
want to use that feature" ... This is a basic engineering requirement.

> paravirt, patching
> ezr       Linux 2.6.21- 1000 0.25 0.53 31.8 34.4 10.1 1.04 5.44 730. 1583 4600
> ezr       Linux 2.6.21- 1000 0.26 0.55 32.1 35.2 13.3 1.03 5.48 748. 1589 4606
> ezr       Linux 2.6.21- 1000 0.26 0.54 32.0 34.9 14.1 1.04 5.43 752. 1606 4647

i guess this pretty much makes the case for patching ...

	Ingo
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