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Date: Thu, 8 May 2008 06:56:45 -0700
From: Arjan van de Ven <arjan@infradead.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Andi Kleen <andi@firstfloor.org>, Matthew Wilcox <matthew@wil.cx>,
       LKML <linux-kernel@vger.kernel.org>,
       Alexander Viro <viro@ftp.linux.org.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [patch] speed up / fix the new generic semaphore code (fix AIM7
 40% regression with 2.6.26-rc1)
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On Thu, 8 May 2008 14:01:30 +0200
Ingo Molnar <mingo@elte.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