linux-kernel - Re: [patch] speed up / fix the new generic semaphore code (fix AIM7 40% regression with 2.6.26-rc1)

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Message-ID: <20080508065645.678dda33@infradead.org>
Date:	Thu, 8 May 2008 06:56:45 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	LKML <linux-kernel@...r.kernel.org>,
	Alexander Viro <viro@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [patch] speed up / fix the new generic semaphore code (fix AIM7
 40% regression with 2.6.26-rc1)

On Thu, 8 May 2008 14:01:30 +0200
Ingo Molnar <mingo@...e.hu> wrote:
> 
> The contention comes from the following property of the new semaphore 
> code: the new owner owns the semaphore exclusively, even if it is not 
> running yet.
> 
> So if the old owner, even if just a few instructions later, does a 
> down() [lock_kernel()] again, it will be blocked and will have to
> wait on the new owner to eventually be scheduled (possibly on another
> CPU)! Or if another other task gets to lock_kernel() sooner than the
> "new owner" scheduled, it will be blocked unnecessarily and for a
> very long time when there are 2000 tasks running.

ok sounds like I like the fairness part of the new semaphores (but
obviously not the 67% performance downside; I'd expect to sacrifice a
little performance.. but this much??).

It sucks though; if this were a mutex, we could wake up the owner of
the bugger in the contented acquire path synchronously.... but these 
are semaphores, and don't have an owner ;( bah bah bah
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