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

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Date:	Thu, 25 Sep 2008 12:23:15 -0400
From:	Mathieu Desnoyers <compudj@...stal.dyndns.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Martin Bligh <mbligh@...gle.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Martin Bligh <mbligh@...igh.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com, "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

* Ingo Molnar (mingo@...e.hu) wrote:
> 
> * Martin Bligh <mbligh@...gle.com> wrote:
> 
> > > And I don't think normalizing later is in any way more fundamentally 
> > > hard. It just means that you do part of the expensive things after 
> > > you have gathered the trace, rather than during.
> > 
> > Agree with you on doing the expensive stuff later. If we wanted to get 
> > something that'd pack down to a couple fewer bits, and approximate ns, 
> > we could always >> 1 if you were > 2GHz, and >> 2 if you where > 4GHz, 
> > etc. which is at least cheap.
> 
> ... which is exactly what sched_clock() does, combined with a 
> multiplication. (which is about as expensive as normal linear 
> arithmetics on most CPUs - i.e. in the 1 cycle range)
> 
> Normalizing has the advantage that we dont have to worry about it ever 
> again. Not about a changing scale due to cpufreq, slowing down or 
> speeding up TSCs due to C2/C3. We have so much TSC breakage all across 
> the spectrum that post-processing it is a nightmare in practice. Plus we 
> want sched_clock() to be fast anyway.
> 
> in the distant future we not only will have constant-TSC but it wont 
> stop in C2/C3 either at a whim (which they do right now, messing up 
> timestamps). At that stage fast time readout it will be so sane that CPU 
> makers should really provide a nanosec readout - it's easy to do a 
> simple multiplicator and hide the few cycles multiplicator latency to 
> RDTSC (this is continuous time after all so it's easy for the hw).
> 
> Hm?
> 
> 	Ingo
> 

Hi Ingo,

The problem with sched_clock is that it gives a 1 HZ timestamp accuracy
for events happening across different CPUs. Within this 1 HZ range, it
uses the TSC and clip when it reaches a max. Good enough for scheduler
or for tracing events on a single CPU, but I think it is not exactly
what we need to reorder events happening across CPUs.

Mathieu

-- 
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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