linux-kernel - Re: [RFC][PATCH 0/9] perf trace: support for general-purpose scripting

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Message-ID: <20091006135311.GB18365@elte.hu>
Date:	Tue, 6 Oct 2009 15:53:11 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tom Zanussi <tzanussi@...il.com>, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, rostedt@...dmis.org, lizf@...fujitsu.com,
	hch@...radead.org
Subject: Re: [RFC][PATCH 0/9] perf trace: support for general-purpose
	scripting


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, 2009-10-06 at 11:09 +0200, Ingo Molnar wrote:
> > * Tom Zanussi <tzanussi@...il.com> wrote:
> > 
> > > Known problems/shortcomings:
> > > 
> > > Probably the biggest problem right now is the sorting hack I added as 
> > > the last patch.  It's just meant as a temporary thing, but is there 
> > > because tracing scripts in general want to see events in the order 
> > > they happened i.e. timestamp order. [...]
> > 
> > Btw., have you seen the -M/--multiplex option to perf record? It 
> > multiplexes all events into a single buffer - making them all ordered. 
> > (The events are in causal ordering in this case even if there's some TSC 
> > asynchronity)
> 
> It also wrecks large machines.. [...]

With millions of events per sec, for sure. It doesnt with a few thousand 
per sec. Right now that's the price of guarantee causality. If you _can_ 
trust your system-wide TSC then it's not needed - but that's only 
possible on a very small subset of machines currently.

> [...] I've been thinking about limiting the number of CPUs you can 
> redirect into a single output stream using the output_fd thing, but 
> then the inherited stuff makes that very hard.
> 
> And we also need a solution for the inhertited counters, the best 
> would be the per-cpu inherited things, where we use both cpu and pid, 
> instead of either.
> 
> In short, -M is nice, but it also has significant down sides, esp. 
> with machines getting more and more cores.

Yeah.

	Ingo
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