Message-ID: <4A2EF833.6080300@zytor.com>
Date: Tue, 09 Jun 2009 17:02:59 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Ray Lee <ray-lk@madrabbit.org>
CC: Andrew Morton <akpm@linux-foundation.org>, renton@renton.name,
       linux-kernel@vger.kernel.org
Subject: Re: devpts mounts too slowly
References: <20090603142801.GC21646@beaver.vrungel.ru> <4A26EB18.6060907@zytor.com> 	<20090609161945.811cc01f.akpm@linux-foundation.org> <2c0942db0906091630l1cc79647h8689e9e929ed1bba@mail.gmail.com>
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Ray Lee wrote:
>>
>> akpm2:/home/akpm/xx# time mount -n -i -t devpts none /tmp/pts
>> mount -n -i -t devpts none /tmp/pts  0.00s user 0.00s system 0% cpu 0.002 total
> 
> I read the Original Post as that test being performed with 5k /dev/pts
> already mounted.

A quick script seems to confirm that the time starts creeping up as the
number of mount points increase; on my Core i7 system the 10000th mount
takes around 50 ms whereas the early instances are submillisecond, and
the increase *seems* to be O(n log n) or thereabouts -- slightly
superlinear but not a good fit for a power law.

The same thing happens with ramfs, so the problem is either in libfs or
in the VFS; I suspect the latter as I don't really think we have had a
huge reason to optimize a very large number of mounts in the past.

	-hpa
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