Re: [PATCH 3/3] perf tools: Make 'struct map_shared' truly shared

[Jiri Olsa]

On Sun, Oct 13, 2019 at 08:10:54PM -0700, Andi Kleen wrote:
> On Sun, Oct 13, 2019 at 05:14:27PM +0200, Jiri Olsa wrote:
> > Andi reported that maps cloning is eating lot of memory and
> > it's probably unnecessary, because they keep the same data.
> > 
> > Changing 'struct map_shared' to be a pointer inside 'struct map',
> > so it can be shared on fork. Changing the map__clone function to
> > actually share 'struct map_shared' for cloned maps.
> > 
> > The 'struct map_shared' carries its own refcnt counter, which is
> > incremented when it's assigned to new 'struct map' and decremented
> > when 'struct map' gets deleted in map__delete (its refcnt is 0).
> > 
> > This 'maps sharing' seems to save lot of heap for reports with
> > many forks/cloned mmaps (over 60% in example below).
> 
> The one case I wasn't sure about is with JIT support. So if
> a map gets modified with fixup/start from /tmp/perf-%d 
> in one process, would it impact the other too?
> 
> We may need a COW operation for this (hopefully rare) case.

so the jitted mmaps are inserted into the data file
and processed during report where they can overload
existing maps - thats detected before addition in:

  thread__insert_map
    map_groups__fixup_overlappings
      - which uses COW way -> map__clone(map, false);
        to create new map

other fixups to maps are being done only for kernel maps,
where we dont have a problem, because there's only one copy

jirka