linux-kernel - Re: devpts mounts too slowly

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Date:	Tue, 7 Jul 2009 00:03:11 +0400
From:	Alexey Vlasov <renton@...ton.name>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Ray Lee <ray-lk@...rabbit.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: devpts mounts too slowly

On Mon, Jul 06, 2009 at 11:52:52PM +0400, Alexey Vlasov wrote:
> On Tue, Jun 09, 2009 at 05:02:59PM -0700, H. Peter Anvin wrote:
> 
> > 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.
> 
> I have prepared a new machine for testing, and I don't see any slowdown
> by mounting devpts.
> Total mounts on it:
> # wc -l /proc/mounts
> 153481 /proc/mounts
> # fgrep -c devpts /proc/mounts
> 10231
> 
> On my other servers there're slowdown already by this number of:
> # wc -l /proc/mounts
> 49889 /proc/mounts
> # fgrep -c devpts /proc/mounts
> 3323

One more notice, box with slowdowns:
...
geteuid()                               = 0
getcwd("/home/staff/r_renton"..., 4095) = 21
readlink("/home/staff/r_renton/none", 0x7ffffa32aa00, 4096) = -1 ENOENT
(No such file or directory)
rt_sigprocmask(SIG_BLOCK, ~[TRAP SEGV RTMIN RT_1], NULL, 8) = 0
mount("none", "/home/www/server_7e014b11/dev/pts",
"devpts", MS_MGC_VAL, NULL) = 0
getcwd("/home/staff/r_renton"..., 4095) = 21
...

And this on the new one, about 100k mounting points and no
slowdowns at all.
...
getuid()                                = 0
geteuid()                               = 0
rt_sigprocmask(SIG_BLOCK, ~[TRAP SEGV RTMIN RT_1], NULL, 8) = 0
mount("none", "/home/www/server_56d48c7d/dev/pts", "devpts",
MS_MGC_VAL, NULL) = 0
readlink("/home", 0x7fff79a82e10, 4096) = -1 EINVAL (Invalid argument)
...

-- 
BRGDS. Alexey Vlasov.
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