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

[Florian Weimer <fw@deneb.enyo.de>]

* 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.

I expect each heap will only allocate two pages via page faults, but
will reserve HEAP_MAX_SIZE bytes of address space.

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.