Why process memory starts so high up in virtual space with FreeBSD malloc?

Giorgos Keramidas keramida at freebsd.org
Tue Dec 2 03:42:42 PST 2008

On Mon, 01 Dec 2008 14:57:23 -0800, Yuri <yuri at rawbw.com> wrote:
> Giorgos Keramidas wrote:
>> The FreeBSD malloc(3) implementation can use either mmap() or sbrk() to
>> obtain memory from the system.  It does not 'waste a high percentage of
>> memory' but it simply maps only high addresses (with an unmapped 'hole'
>> in lower addresses).
> But the hole it leaves with MALLOC_OPTIONS='dM' is way larger than the
> one left by 'Dm' option. Usually malloc will keep allocating addresses
> higher than this initial value and will never come back and fill some
> parts of this gap.  Therefore "wasting" this space.

The 'D' and 'M' options set what malloc() will _prefer_, they do not
force malloc() to use _only_ the particular type of memory space.  As
Dan explained in another post, both memory types will be used if there
is need for more address space.

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