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Date: Mon, 23 Sep 2002 00:55:06 +0200 (CEST)
From: Ingo Molnar <mingo@elte.hu>
Reply-To: Ingo Molnar <mingo@elte.hu>
To: bob <bob@watson.ibm.com>
Cc: Karim Yaghmour <karim@opersys.com>, <okrieg@us.ibm.com>, <trz@us.ibm.com>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       LTT-Dev <ltt-dev@shafik.org>, Linus Torvalds <torvalds@transmeta.com>
Subject: Re: [ltt-dev] Re: [PATCH] LTT for 2.5.38 1/9: Core infrastructure
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On Sun, 22 Sep 2002, bob wrote:

> > (this is in essence a moving spinlock at the tail of the trace buffer -
> > same problem.)
> 
> No, we use lock-free atomic operations to reserve a place in the buffer
> to write the data.  What happens is you attempt to atomic move the
> current index pointer forward.  If you succeed then you have bought
> yourself that many data words in the queue.  In the unlikely event you
> happened to collide with someone you perform the atomic operation again.

you have not understood what i have written.

what you do has the same (bad) effect as a global spinlock, it in essence
has the same cache effect as a constantly moving spinlock at the 'end' of
the trace buffer. Cachelines bounce between CPUs. Only completely per-CPU
trace buffers solve this problem.

	Ingo

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