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

[ritesh sonawane <rdssonawane2317 at gmail dot com>]

On Thu, Sep 27, 2018 at 11:15 PM Florian Weimer <fw@deneb.enyo.de> 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.
>
>
Thank you for more clarification. I have set DEFAULT_MMAP_THRESHOLD_MAX  to
1GB
and HEAP_MAX_SIZE kept default (2 * DEFAULT_MMAP_THRESHOLD_MAX) and
DEFAULT_MMAP_THRESHOLD is also set to DEFAULT_MMAP_THRESHOLD_MAX instead of
DEFAULT_MMAP_THRESHOLD_MIN, So that default value for M_MMAP_THRESHOLD will
be 1GB.
Because we want default value of M_MMAP_THRESHOLD to be 1GB. With this
changes we got the
improvement in memory utilization (less fragmentation).
But having some doubt regarding 32GiB HEAP_MAX_SIZE. Because we want
malloc() request above
1GB to be done by mmap()  to keep minimum memory locked by arena.

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

Currently we are not supporting the demand paging.

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.
>

Sure, I will try this also,