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

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

> On Tue, 2009-10-06 at 11:09 +0200, Ingo Molnar wrote:
> > * Tom Zanussi <tzanussi@gmail.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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