Impact of Increasing M_MMAP_THRESHOLD to 1GB in malloc().

[ritesh sonawane]

On Thu, Sep 27, 2018 at 11:45 PM Carlos O'Donell <carlos@redhat.com> wrote:

> On 9/27/18 1:45 PM, Florian Weimer wrote:
> > * ritesh sonawane:
> >
> >> Yes it is 64 Bit target.
> >>
> >> Fragmentation means when malloc() request is more than threshold
> >> value, memory is allocated using mmap().  Due to size alignment with
> >> page size, there is memory wastage per request.  e.g. Our system is
> >> having 48GB memory, then total Number of malloc() requests (each
> >> 64MB) will be 380 and total used memory is 23 GB out of 48GB.
> >
> > I see.  I agree that's a problem, and changing the malloc threshold
> > could be a solution.
> >
> >> The system (NEC SX-Aurora TSUBASA) on which we are currently working
> >> is having huge page size and each process can have 16 GB and 512 GB
> >> address space for 2 MB and 64 MB page size respectively.
> >
> > I'm not familiar with that system and haven't seen the glibc port,
> > sorry.  I used to work next door to a NEC SX-6 as a student, but
> > that's it.
> >
> >> Also there is no particular limit on maximum heap size and Each
> >> thread can easily consumes 2 GB of address space and thats why we
> >> want to increase the M_MMAP_THRESHOLD to 1 GB.
> >
> > If you do that, you also have to increase the heap size to something
> > like 32 GiB (HEAP_MAX_SIZE in malloc/arena.c).  The default of 2 *
> > DEFAULT_MMAP_THRESHOLD_MAX is probably too small (assuming that
> > DEFAULT_MMAP_THRESHOLD_MAX will be 2 GiB).  Otherwise you will have
> > substantial fragmentation for allocation requests between 2 GiB and
> > HEAP_MAX_SIZE.
>
> Right.
>
> * Change malloc/malloc.c DEFAULT_MMAP_TRESHOLD_MAX to 16GiB.
> * This in turn sets HEAP_MAX_SIZE to 32GiB.
> * Now you can set M_MMAP_THRESHOLD to any value 0 > x <= (32GiB - a couple
> of pages).
>
> Thanks for suggestions,  Right now requirement of M_MMAP_THRESHOLD is Up
to 1GB only.
So I set DEFAULT_MMAP_TRESHOLD_MAX to 1GB ( for 2MB and 64MB page size) and
testing
the performance. But I will check with above limits also.

Apart from changing threshold value do I need to take care of bin index ?
I have tried to understand the binning of malloc(), and found that its not
necessary.

The key here is that an arena is made up of discontiguous heaps in
> a chain which are logically "the heap" for the attached thread (non-main
> arena). You can't have a request that crosses heap boundaries, so if
> you set M_MMAP_THRESHOLD larger than the heap size it is impossible to
> service the request except through mmap.
>
> All of this requires a custom glibc.
>
> I *had* some patches to set HEAP_MAX_SIZE for testing, but it can only
> be set at startup via a tunable because once set all allocations have
> to use it for masking to compute chunk->heap mapping.
>
> I never had a reason to change it... but this example is such a reason.
>
> I don't think it's a terrible idea to allow a tunable for HEAP_MAX_SIZE
> or DEFAULT_MMAP_THRESHOLD_MAX, but can only be set early and not late,
> which is fine.
>
> > I expect each heap will only allocate two pages via page faults, but
> > will reserve HEAP_MAX_SIZE bytes of address space.
>
> Agreed.
>
> > If that's a problem, you could also make these changes, but set the
> > maximum arena count to 1, then only the main arena will be used, which
> > is sbrk-based.  The main arena doesn't need the coarse-grained
> > mappings with large power-of-two sizes.
>
> Right.
>
> --
> Cheers,
> Carlos.
>

Thanks and Best Regards,
Ritesh Sonawane