[Bug lto/50511] New: gcc lto streamer in fragments memory badly

["andi-gcc at firstfloor dot org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50511

             Bug #: 50511
           Summary: gcc lto streamer in fragments memory badly
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: andi-gcc@firstfloor.org


I ran into a problem while testing LTO on 
a quite large project with a lot of object files:

the lto streamer fragmented the memory map badly by constantly mapping
and unmapping the input files with mmap. I ended up with a memory
map with lots of 2 page holes between mappings. Eventually it bumped
into the default 64k max number of mappings limit on Linux and errored out
because mmap failed.

Workaround was to increase this limit (sysctl -w vm.max_map_count = 200000)
However gcc should be more efficient in its mappings.

I think the problem is the one off cache in lto_file_read() being too dumb.
Looking into a fix.