linux-kernel - Re: devpts mounts too slowly

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Message-ID: <4A2EF833.6080300@zytor.com>
Date:	Tue, 09 Jun 2009 17:02:59 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Ray Lee <ray-lk@...rabbit.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>, renton@...ton.name,
	linux-kernel@...r.kernel.org
Subject: Re: devpts mounts too slowly

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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