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

[Andi Kleen]

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.

-Andi