linux-kernel - Re: [RFC PATCH 1/2 v2] Unified trace buffer

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Message-ID: <alpine.LFD.1.10.0809251023090.3265@nehalem.linux-foundation.org>
Date:	Thu, 25 Sep 2008 10:25:00 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/2 v2] Unified trace buffer



On Thu, 25 Sep 2008, Steven Rostedt wrote:
> 
> The problem with this is overwrite mode, which is the only mode ftace 
> currently offers. What happens when your writer starts overwriting the 
> ring buffer and there is no reader?

Overwrite things on page at a time. Don't you already do that? (I didn't 
check that closely, I just assumed you would do the _much_ simpler "move 
the head to the next page" thing rather than trying to mix head and tail 
on the same page.

> What happens is that the start value is gone. You do not have a way to use 
> all the deltas to catch up to the remaining events.

Use the page start date for the first event in a page. But within pages, 
make everything depend on previous event.

		Linus
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