linux-kernel - Re: aim7 performance regression by commit 5a50508 report from LKP

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Date:	Tue, 29 Jan 2013 10:12:45 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Yuanhan Liu <yuanhan.liu@...ux.intel.com>
Cc:	Huang Ying <ying.huang@...el.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	linux-kernel@...r.kernel.org, lkp@...ux.intel.com
Subject: Re: aim7 performance regression by commit 5a50508 report from LKP


* Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:

> On Tue, Jan 29, 2013 at 09:44:00AM +0100, Ingo Molnar wrote:
> > 
> > * Yuanhan Liu <yuanhan.liu@...ux.intel.com> wrote:
> > 
> > > [...]
> > 
> > Very nice measurements and analysis, thanks!
> > 
> > > As stated above, anybody can have a chance to own the lock in 
> > > mutex once somebody release the lock. Well, there is only one 
> > > to own the lock in rwsem write lock, and the one is known 
> > > already: the one in the head of wait list. That would result 
> > > to more contention in rwsem write lock case, especially if the 
> > > one _will_ own the lock is not running now.
> > 
> > I think we should allow lock-steal between rwsem writers - that 
> > will not hurt fairness as most rwsem fairness concerns relate to 
> > reader vs. writer fairness.
> 
> Agreed, and I'm sure this will improve performance and may 
> make this performance regression go away.
> 
> David, is that Ok to you? If so, I may have a try.

I'm not David but please try it :-)

Making rwsem behavior and scalability similar to mutexes would 
have numerous advantages.

> > Am I correct to assume that all relevant users in this 
> > workload are down_write() users?
> 
> Yes, as commit 5a50508 just convert all mutex to down_write.

A second track of inquiry would be to see whether any of the key 
usage sites could be converted to down_read() or whether the 
lock hold times could be reduced drastically - but I doubt 
that's really possible on such heavily forking workloads.

Thanks,

	Ingo
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