linux-kernel - Re: [RFC PATCH 1/3] Unified trace buffer

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Date:	Wed, 24 Sep 2008 17:08:33 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Martin Bligh <mbligh@...gle.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	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,
	Tom Zanussi <zanussi@...cast.net>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [RFC PATCH 1/3] Unified trace buffer


On Wed, 24 Sep 2008, Martin Bligh wrote:

> > On Wed, 24 Sep 2008, Martin Bligh wrote:
> >>
> >> If we use 32 bits instead of 27, then the timestamp events are only
> >> about once per second, which is probably fine for overhead ... ?
> >
> > You'd have them ONCE IN A BLUE MOON.
> >
> > If there is nothing going on, you don't need the timestamps at all.
> 
> Yeah, you're right - we can just mark it dirty, and 'pre-log' the timestamp
> events when someone calls a reserve and we haven't logged anything
> for more time than we can store. Did not think of that. Was only 5 bits
> for us, not an extra 37, but still, is much better.
> 
> Is a 5-bit event id generic enough though?
> 

Actually, I was keeping the event id completely out of the ring buffer and 
let a higher layer deal with that. For padding, I just made the length 
field zero.

For overflows of the timestamp, we can reserve the -1 timestamp as a 
trigger to read the tsc again and put the full 64 bits into the record.

Just an idea.

-- Steve

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