Re: [RFC/PATCH] perf record: Save build-id of DSO in callchains

[Namhyung Kim <namhyung@kernel.org>]

Hi Ingo,

On Mon, Mar 23, 2015 at 08:22:14AM +0100, Ingo Molnar wrote:
> 
> * Namhyung Kim <namhyung@kernel.org> wrote:
> 
> > Currently the build-id only recorded for sampled location, but in
> > order to correctly view/annotate callchains it might need the
> > build-id's of callchains too.  I guess this choice was due to a
> > performance impact on the post-processing at perf record time.
> > 
> > Add a new option --buildid-callchain to record this info.
> > 
> > $ perf record -o xxx -g -- perf > /dev/null
> > $ perf buildid-list -i xxx
> > 6ebcee76c4b04895598b4df86ec445c49fc137a2 /lib/modules/3.18.6-1-ARCH/build/vmlinux
> > fc0759b71584d2513b04f1b530965a61b5e499e2 /usr/lib/ld-2.21.so
> > 
> > $ perf record -o yyy -g --buildid-callchain -- perf > /dev/null
> > $ perf buildid-list -i yyy
> > 6ebcee76c4b04895598b4df86ec445c49fc137a2 /lib/modules/3.18.6-1-ARCH/build/vmlinux
> > 4235ae90856f98d2746529648a3339ebfa6ede43 /home/namhyung/project/linux/tools/perf/perf
> > fc0759b71584d2513b04f1b530965a61b5e499e2 /usr/lib/ld-2.21.so
> > 9ac81172d5ff96f40d984fe7c10073a98f1a6b2e /usr/lib/libc-2.21.so
> > 864583bb881db1d260660a30a51b3f5022cd538b /usr/lib/libnuma.so.1.0.0
> 
> Just curious, could you try to measure the performance impact of this 
> change?

  $ time perf record --call-graph dwarf -o kbuild.xxx -a -- sleep 30
  [ perf record: Woken up 34674 times to write data ]
  [ perf record: Captured and wrote 9538.729 MB kbuild.xxx (1211997 samples) ]

  real   0m32.244s
  user   0m1.097s
  sys    0m8.997s

  $ time perf record --call-graph dwarf -o kbuild.yyy --buildid-callchain -a -- sleep 30
  [ perf record: Woken up 40902 times to write data ]
  [ perf record: Captured and wrote 11208.500 MB kbuild.yyy (1433922 samples) ]

  real   2m21.695s
  user   1m33.127s
  sys    0m22.077s

> 
> Also, unless the performance (or file size) effect is horrible, I 
> think this flag should be implicitly set by -g.

Agreed.  But unfortunately, it seems horrible. ;-p

> 
> 99.999% of users won't know about this flag, and will see broken 
> annotations!

Well, most of them will see correct one unless the binary is changed
in the meantime. :)

Thanks,
Namhyung