3.2.2 The GNU Allocator

The 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 threshold for 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 mmap can be disabled completely with M_MMAP_MAX; 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.

It is possible to use your own custom malloc instead of the built-in allocator provided by the GNU C Library. See Replacing malloc.