On Mon, Sep 19, 2016 at 11:04 AM, Jeroen Demeyer <jdeme...@cage.ugent.be> wrote:
> On 2016-09-19 11:54, Jonathan Bober wrote: > >> But in that case the memory does effectively get "used", if it is the >> case that a quarter of the machine's memory is only available from the >> PARI memory allocater. If I'm not using the PARI parts of Sage, is that >> memory completely wasted? >> > > There should not be any consequences apart from the artificially high > value of virtual memory used. It certainly does not affect the physical > memory or swap space which is effectively available. > So the question becomes: what uses the "virtual memory used" value? > Apparently, the Linux overcommit thing does use that value (which is > surprising to me, I never really understood how this overcommit stuff > works). > > If I ulimit -v 8 GB, say, (which is 512/64), and the PARI allocater immediately grabs 2 GB of the virtual address space for itself, then that seems like it leaves only 6 GB for malloc/sage_malloc/whatever else, which would be effectively limiting the physical memory available in some situations. This specific machine has overcommit_memory set to 2, which means that the total amount of non-file-backed writable virtual memory that the kernel will allocate is fixed (probably to somewhere around the physical size of RAM). This is not the default, and you probably wouldn't want to run your laptop this way, but is not so unreasonable for a shared computer with a lot of RAM, where it usually doesn't cause problems. (It's a setting that might get changed from the default after the first or second time someone crashes the machine by using too much memory, especially if this happens on a Friday afternoon and leaves the machine unavailable all weekend...) When it does cause problems, it's usually because a process with a lot of writable virtual memory allocated to it forks, as is happening here. -- You received this message because you are subscribed to the Google Groups "sage-support" group. To unsubscribe from this group and stop receiving emails from it, send an email to sage-support+unsubscr...@googlegroups.com. To post to this group, send email to sage-support@googlegroups.com. Visit this group at https://groups.google.com/group/sage-support. For more options, visit https://groups.google.com/d/optout.
