|Bill Allombert on Mon, 03 Oct 2016 20:21:57 +0200|
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|Re: Use PROT_NONE for unused virtual stack memory|
On Mon, Oct 03, 2016 at 05:04:53PM +0200, Jeroen Demeyer wrote: > On 2016-09-29 17:46, Bill Allombert wrote: > >Do you have some framework to test this ? > > Not really because it depends a lot on the OS. In order to properly test > this, you need a Linux system with vm.overcommit = 2. So you need > > # echo 2 >/proc/sys/vm/overcommit_memory What linux kernel version did you test ? And how did you test it ? > >>The idea is to use PROT_NONE for the unused part of the PARI stack (between > >>vbot and bot). Memory mmap()ed with PROT_NONE is not committed: it does not > >>count against the available physical/swap memory available for the system. > > > >Do you have some relevant documentation for this behaviour ? > > No. As far as I know, this is undocumented. This is problematic... I wrote this code following the POSIX standard. The code is not meant to be linux specific. However I have no issue with using map(,PROT_NONE) and remapping to map(,PROT_WRITE|PROT_READ) if it helps since this is POSIX compliant. Maybe pari_mainstack_malloc could take both size in arguments and do both mmap at once. Did you try madvise(, MADV_DONTNEED) ? > But the trick is known and used > in the wild, see for example the discussion > > https://github.com/jemalloc/jemalloc/issues/255 which links to <https://github.com/jemalloc/jemalloc/issues/193> which says: " The overcommit heuristic is clearly not sensible when using jemalloc, so I think it would make sense to detect it and set MAP_NORESERVE on all of the mappings. Rather than catching obvious errors, it's resulting in an inability to use all of (well, more than half of) the system's resources. Alternatively, MAP_NORESERVE could just be set by default. It's a no-op for the other 2 overcommit states (it's not documented correctly in the man pages project). " But your patch remove MAP_NORESERVE. Why ? Cheers, Bill.