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From: Adrian Chadd <adrian@freebsd.org>
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Cc: Garrett Cooper <yanegomi@gmail.com>, freebsd-performance@freebsd.org,
	Dieter BSD <dieterbsd@engineer.com>
Subject: Re: cmp(1) has a bottleneck, but where?
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On 3 January 2012 00:34, Marc Olzheim <marcolz@stack.nl> wrote:
> On Tue, Jan 03, 2012 at 12:21:10AM -0800, Garrett Cooper wrote:
>> =A0 =A0 The file is 3.0GB in size. Look at all those page faults though!
>> Thanks!
>> -Garrett
>
> From usr.bin/cmp/c_regular.c:
>
> #define MMAP_CHUNK (8*1024*1024)
> ...
> for (..) {
> =A0 =A0 =A0 =A0mmap() chunk of size MMAP_CHUNK.
> =A0 =A0 =A0 =A0compare
> =A0 =A0 =A0 =A0munmap()k
> }
>
> That 8 MB chunk size sounds like a bad plan to me. I can imagine
> something needed to be done to compare files larger than X GB on a 32bit
> system, but 8MB is pretty small...

Er, hint: look at the average IO size in the cmp versus cp stats above?

Something/somehow it's issuing smaller IOs when using mmap?


Adrian