linux-kernel - Re: -rt dbench scalabiltiy issue

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Date:	Tue, 24 Nov 2009 18:16:17 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhltc@...ibm.com>,
	Clark Williams <williams@...hat.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: -rt dbench scalabiltiy issue

On Mon, 2009-11-23 at 10:06 +0100, Nick Piggin wrote:
> BTW. Have you tested like ext4, xfs and btrfs cases?  I don't think ext3
> is likely to see a huge amount of scalability work, and so it will be
> good to give an early heads up to the more active projects...

Yea, I need to give those a shot. I also generated the same numbers as
before with ext2 (all the raw numbers are in dbench-scalability dir):

http://sr71.net/~jstultz/dbench-scalability/graphs/ext2-scalability.png

Again, its similar to ext3, in that all the -rt variants are hitting
some contention issues. But I was a little surprised to see
2.6.32-rc7-nick below 2.6.32-rc7 there, so generated perf data there as
well:

http://sr71.net/~jstultz/dbench-scalability/perflogs/2.6.32-rc7-nick.ext2.perflog


31.91%       dbench  [kernel]                    [k] _spin_lock
                |          
                |--44.17%-- dput
                |          |          
                |          |--50.39%-- __link_path_walk
                |          |          |          
                |          |          |--99.88%-- path_walk
                |          |          |          |          
                |          |          |          |--96.18%-- do_path_lookup
                |          |          |          |          |          
                |          |          |          |          |--51.49%-- user_path_at
                |          |          |          |          |          |          
                |          |          |          |          |          |--96.84%-- vfs_fstatat
                |          |          |          |          |          |          vfs_stat
                |          |          |          |          |          |          sys_newstat
                |          |          |          |          |          |          system_call_fastpath
                |          |          |          |          |          |          _xstat
                |          |          |          |          |          |          
                |          |          |          |          |           --3.16%-- do_utimes
                |          |          |          |          |                     sys_utime
                |          |          |          |          |                     system_call_fastpath
                |          |          |          |          |                     __GI_utime
                |          |          |          |          |          
                |          |          |          |          |--43.18%-- do_filp_open
                |          |          |          |          |          do_sys_open
                |          |          |          |          |          sys_open
                |          |          |          |          |          system_call_fastpath
                |          |          |          |          |          __GI___open
...

		|--39.98%-- path_get
                |          |          
                |          |--51.19%-- path_init
                |          |          |          
                |          |          |--96.40%-- do_path_lookup
                |          |          |          |          
                |          |          |          |--55.12%-- user_path_at
                |          |          |          |          |          
                |          |          |          |          |--91.47%-- vfs_fstatat
                |          |          |          |          |          vfs_stat
                |          |          |          |          |          sys_newstat
                |          |          |          |          |          system_call_fastpath
                |          |          |          |          |          _xstat
                |          |          |          |          |          |          
                |          |          |          |          |           --100.00%-- 0x7478742e746e65
                |          |          |          |          |          


So I'm a little confused here. Why is dput/path_get showing up in ext2
when it all but disappeared with ramfs? Does ramfs have some sort of
vfs_stat shortcut that avoids whatever is catching us here?

thanks
-john

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