Q61313: malloc() Is Slower in Large and Compact Models

Article: Q61313
Product(s): See article
Version(s): 5.00 5.10 6.00 | 5.10 6.00
Operating System(s): MS-DOS | OS/2
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 11-JUL-1990

In LARGE and COMPACT models, it should be expected that malloc will be
slower since it must use far pointer calls. It may be possible to
improve malloc's performance by adjusting the _amblksiz variable to an
appropriate size.

The _amblksiz variable can be used to control the amount of memory
space in the heap used by C for dynamic memory allocation. This
variable is declared in the include file MALLOC.H.

The first time your program calls one of the dynamic memory allocation
functions (such as calloc or malloc), it asks the operating system for
an initial amount of heap space that is typically much larger than the
amount of memory requested by calloc or malloc. This amount is
indicated by _amblksiz, whose default value is 8K (8192 bytes).
Subsequent memory allocations are allotted from this 8K of memory,
resulting in fewer calls to the operating system when many relatively
small items are being allocated. C calls the operating system again
only if the amount of memory used by dynamic memory allocations
exceeds the currently allocated space.

If the requested size in your C program is greater than _amblksiz,
multiple blocks, each of size _amblksiz, are allocated until the
request is satisfied. Since the amount of heap space allocated is more
than the amount requested, subsequent allocations can cause
fragmentation of heap space. You can control this fragmentation by
using _amblksiz to change the default memory chunk to whatever value
you like, as in the following example:

   _amblksiz = 2000;

Since the heap allocator always rounds the DOS request to the nearest
power of 2 greater than or equal to _amblksiz, the preceding statement
causes the heap allocator to reserve memory in the heap in multiples
of 2K (2048 bytes).

Note that adjusting the value of _amblksiz affects both near- and
far-heap allocation. Adjusting this value has no effect on halloc or
_nmalloc in any memory model.

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