malloc implementation in the GNU C Library is derived from ptmalloc
(pthreads malloc), which in turn is derived from dlmalloc (Doug Lea malloc).
This malloc may allocate memory in two different ways depending on their size
and certain parameters that may be controlled by users. The most common way is
to allocate portions of memory (called chunks) from a large contiguous area of
memory and manage these areas to optimize their use and reduce wastage in the
form of unusable chunks. Traditionally the system heap was set up to be the one
large memory area but the GNU C Library
malloc implementation maintains
multiple such areas to optimize their use in multi-threaded applications. Each
such area is internally referred to as an arena.
As opposed to other versions, the
malloc in the GNU C Library does not round
up chunk sizes to powers of two, neither for large nor for small sizes.
Neighboring chunks can be coalesced on a
free no matter what their size
is. This makes the implementation suitable for all kinds of allocation
patterns without generally incurring high memory waste through fragmentation.
The presence of multiple arenas allows multiple threads to allocate
memory simultaneously in separate arenas, thus improving performance.
The other way of memory allocation is for very large blocks, i.e. much larger
than a page. These requests are allocated with
mmap (anonymous or via
/dev/zero; see Memory-mapped I/O)). This has the great advantage
that these chunks are returned to the system immediately when they are freed.
Therefore, it cannot happen that a large chunk becomes “locked” in between
smaller ones and even after calling
free wastes memory. The size
mmap to be used is dynamic and gets adjusted according to
allocation patterns of the program.
mallopt can be used to statically
adjust the threshold using
M_MMAP_THRESHOLD and the use of
can be disabled completely with
see Malloc Tunable Parameters.
A more detailed technical description of the GNU Allocator is maintained in the GNU C Library wiki. See https://sourceware.org/glibc/wiki/MallocInternals.