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Message-ID: <43368887.2020008@yahoo.com.au>
Date: Sun, 25 Sep 2005 21:22:47 +1000
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Tejun Heo <htejun@gmail.com>
CC: viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org
Subject: Re: [PATCH linux-2.6 01/04] brsem: implement big reader semaphore
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Tejun Heo wrote:

>  I did a busy-loop microbenchmark, and I think it's informative enough. :-)
> 

Great!

>  The following command is run on three versions - vanilla version, one 
> with read_down/up(->s_umount) added to vfs_write(), and one with 
> brsem_read_down/up(->s_umount) added to vfs_write().
> 
> # time -p dd if=/dev/zero of=out bs=32 count=10M
> 
>  The test is run three times and the results are averaged.
> 
> a. vanilla
> 
> real 58.63
> user 5.61
> sys 52.37
> 
> b. rwsem
> 
> real 59.24
> user 6.06
> sys 52.29
> 
> c. brsem
> 
> real 61.74
> user 5.78
> sys 55.04
> 
>  I don't think brsem has any chance of being faster than rwsem if it's 
> slower in this micro benchmark.  One weird thing is that the result of 
> rwsem consistently showed higher user time and lower system time than 
> vanilla (no synchronization) case, maybe oprofiling will tell something.
> 

Yep, probably just some timing or cache anomaly, generally just
sum the user and system time in the case where you are running
identical userspace code... I wouldn't worry too much about it.

>  Anyways, you were absolutely right.  My brsem was a pretty stupid idea 
> after all.  Let's hope at least I learned something from it.  :-(
> 

I wouldn't say stupid. Implementation was too complex, but some
clever ideas were required to solve the problems you identified.

There are definitely a couple of possible places where a brsem
may be useful, and I think cpucontrol semaphore is one of those.

Al can probably comment on its use in the superblock. It seems
fair enough, though I think we'll want to slim down my naive
rwsem array and maybe think about using alloc_percpu.

Is the remount case much rarer than mount/unmount? Do we need to
be careful about bloating the size of the superblock on large
machines? In that case maybe a global remount brsem will be
enough to handle this race?

-- 
SUSE Labs, Novell Inc.

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