[Bug other/50636] New: GC in large LTO builds cause excessive fragmentation in memory map

[Jan Hubicka]

> When doing a very large LTO build I fail with "out of virtual memory"
> 
> Some investigation showed the problem was not actually running out of 
> memory, but gcc excessively fragmenting its memory map. The Linux kernel
> has a default limit of 64k mappings per process and the fragmentation 
> exceeded that. This lead to gc mmap allocations failing and other problems.
> 
> A workaround is to increase /proc/sys/vm/max_map_count
> 
> Looking at /proc/$(pidof lto1)/maps I see there are lots of 1-3 page holes
> between other anonymousmemory. I think that's caused by ggc-pages free_pages()
> function freeing too early and in too small chunks
> (and perhaps LTO garbage collecting too much?)

In gcc-2.95 times ggc-page was probably not written with 8GB of memory use in
mind :) Perhaps ggc-page should simply increase the chunks as memory grows?
(i.e. release to system only when 20% of memory is unused & it exceeds some minimal
value)

Honza