linux-kernel - Re: [PATCH] Enhance perf to collect KVM guest os statistics from host side

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Date:	Tue, 23 Mar 2010 11:29:27 -0300
From:	Arnaldo Carvalho de Melo <acme@...radead.org>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Soeren Sandmann <sandmann@...mi.au.dk>,
	Joerg Roedel <joro@...tes.org>, Ingo Molnar <mingo@...e.hu>,
	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Sheng Yang <sheng@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Jes Sorensen <Jes.Sorensen@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Zachary Amsden <zamsden@...hat.com>, zhiteng.huang@...el.com,
	Fr??d??ric Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH] Enhance perf to collect KVM guest os statistics from
	host side

Em Tue, Mar 23, 2010 at 03:20:11PM +0100, Andi Kleen escreveu:
> Soeren Sandmann <sandmann@...mi.au.dk> writes:
> > I don't think the oprofile JIT interface solves any of these
> > problems. (In fact, I don't see why the JIT problem is even hard. The
> > JIT compiler can just generate a little ELF file with symbols in it,
> > and the profiler can pick it up through the mmap events that you get
> > through the perf interface).
> 
> That would require keeping those temporary ELF files for
> potentially unlimited time around (profilers today look at the ELF
> files at the final analysis phase, which might be weeks away)

'perf record' will traverse the perf.data file just collected and, if the
binaries have build-ids, will stash them in ~/.debug/, keyed by build-id
just like the -debuginfo packages do.

So only the binaries with hits. Also one can use 'perf archive' to
create a tar.bz2 file with the files with hits for the specified
perf.data file, that can then be transfered to another machine, whatever
arch, untarred at ~/.debug and then the report can be done there.

As it is done by build-id, multiple 'perf record' sessions share files
in the cache.

Right now the whole ELF file (or /proc/kallsyms copy) is stored if
collected from the DSO directly, or the bits that are stored in
-debuginfo files if we find it installed (so smaller). We could strip
that down further by storing just the ELF sections needed to make sense
of the symtab.

- Arnaldo
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