On Mon, Sep 19, 2016 at 11:04 AM, Jeroen Demeyer <jdeme...@cage.ugent.be>

> 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

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.

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