Re: [PATCH 02/26] perf: Introduce mmap3 version of mmap event

[Jiri Olsa]

On Mon, Sep 14, 2020 at 10:26:05AM -0700, Stephane Eranian wrote:
> On Mon, Sep 14, 2020 at 2:08 AM <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Sun, Sep 13, 2020 at 11:41:00PM -0700, Stephane Eranian wrote:
> > > On Sun, Sep 13, 2020 at 2:03 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > > what happens if I set mmap3 and mmap2?
> > >
> > > I think using mmap3 for every mmap may be overkill as you add useless
> > > 20 bytes to an mmap record.
> > > I am not sure if your code handles the case where mmap3 is not needed
> > > because there is no buildid, e.g, anonymous memory.
> > > It seems to me you've written the patch in such a way that if the user
> > > tool supports mmap3, then it supersedes mmap2, and thus
> > > you need all the fields of mmap2. But if could be more interesting to
> > > return either MMAP2 or MMAP3 depending on tool support
> > > and type of mmap, that would certainly save 20 bytes on any anon mmap.
> > > But maybe that logic is already in your patch and I missed it.
> >
> > That, and what if you don't want any of that buildid nonsense at all? I
> > always kill that because it makes perf pointlessly slow and has
> > absolutely no upsides for me.
> >
> I have seen situations where the perf tool takes a visibly significant
> amount of time (many seconds) to inject the buildids at the end of the
> collection
> of perf record (same if using perf inject -b). That is because it
> needs to go through all the records in the perf.data to find MMAP
> records and then read
> the buildids from the filesystem. This has caused some problems in our
> environment. Having the kernel add the buildid to *relevant* mmaps
> would avoid
> a lot of that penalty, by avoiding having to parse the perf.data file
> and leveraging the fact that the buildid may be in memory already.
> Although my concern on
> this has to do with large pages and the impact they have on alignment
> of sections in memory.  I think Ian can comment better on this.
> 
> I think this patch series is useful if it can demonstrate a speedup
> during recording (perf record or perf record | perf inject -b). But it

I haven't meassured, but I assume skipping of perf.data search
at the end of the record will make up for reading buildid for
each mmap event.. migt be tricky in mmap events heavy workloads

> needs to be
> optimized to minimize the volume of useless info returned. And Jiri
> needs to decide if MMAP3 is a replacement of MMAP2, or a different
> kind of record
> targeted at ELF images only in which case some of the fields may be
> removed. My tendency would be to go for the latter.

yes, I like the latter as well, let's see

thanks,
jirka