Re: [RFC/PATCH 00/14] perf record: Add support to store data in directory

[Stephane Eranian]

On Thu, Feb 14, 2019 at 1:35 PM Arnaldo Carvalho de Melo
<arnaldo.melo@xxxxxxxxx> wrote:
>
> On Thu, Feb 14, 2019, 6:31 PM Stephane Eranian <eranian@xxxxxxxxxx wrote:
>>
>> On Thu, Feb 14, 2019 at 6:00 AM Arnaldo Carvalho de Melo
>> <acme@xxxxxxxxxx> wrote:
>> >
>> > Em Thu, Feb 14, 2019 at 02:26:38PM +0100, Jiri Olsa escreveu:
>> > > On Thu, Feb 14, 2019 at 09:57:59AM -0300, Arnaldo Carvalho de Melo wrote:
>> > > > But with the build id in the MMAPs we wouldn't need to scan anything at
>> > > > 'perf record' time, just at 'perf archive' time, where we would scan
>> > > > everything, and as soon as we find a hit, we add that DSO to the list of
>> > > > things we need to put in the tarball.
>> >
>> > > ok.. it might little change the expected behavour in that you will not
>> > > have .debug cache populated until you run perf archive.. some profile
>> > > data might stop report after you reinstall the binary..
>> >
>> Today perf record does collect the buildids once monitor is completed,
>> it does one
>> pass over the perf.data file looking for MMAP records, or at least in
>> the version I am
>> more familiar with.
>
>
> It does look for the MMAP records, how else would it find the DSO pathnames?
>
I was not talking about the synthesize_mmap() code. I was talking
about process_buildids().

>
> - Arnaldo
>
> Sent from smartphone
>>
>>
>> > Well, nothing prevents us from continuing to, in 'perf record', go thru
>> > the perf.data just collected to populate the .debug cache, its just that
>> > we would do it just for that, for populating the cache, we wouldn't
>> > _have_ to do that for collecting the buildids.
>>
>> Sure, for compatibility reasons.
>> In pipe mode, this would also avoid the need for perf inject -b -i -,
>> which is a win.
>>
>> perf archive is useful if you do not have a way to locate the binary using  only
>> the buildid on the analysis machine.
>> >
>> > > on the other hand '.debug' cache would stop growing uncontrolably..
>> > > so I think I'd be ok with this
>> >
>> I agree!
>>
>> > That is another problem, and one that has to be solved in a way similar
>> > to ccache's --max-size option.
>> >
>> > The .debug cache is useful for various workflows, but may get in the way
>> > for some others, which is why we have ways to disable it.
>> >
>> Correct.
>>
>> > For instance, when working on some project it is handy to have copies of
>> > binaries saved so that older perf.data files can find a file that was
>> > rewritten by the compiler when doing changes in it, etc.
>> >
>> > With the .debug cache and if using -g, we can get the older copy of the
>> > binary _and_ its sources, annotation for older versions continue to
>> > work, etc.
>> >
>> > - Arnaldo