Re: Memory fragmentation

"James Kanze" <>
Wed, 28 Feb 2007 05:45:40 CST
Ommund wrote:

We are currently developing an embedded system with a reasonable
amount of RAM. The system is intended to run for years without faults,
restarts etc. We are using the Integrity C++ compiler from Green Hill.

Most of the heap memory is allocated at startup while reading
configuration data, but there are also a certain amount of allocation
during normal operation mostly because messages in form of small
character arrays are sent between different tasks, and because the
std::string type is used internally in the tasks in the processing
external events.

My view is that there is at least a teoretical risk for the system to
crash due to memory fragmentation.

The question is if an allocator that ensures that the size of the
allocated areas are given by 2**k where k is selected from a
reasonable range will prevent fragmentation? And under which
conditions this might be true?

It will reduce the probability of fragmentation considerably, at
a cost of considerably more memory being needed to begin with.
It won't guarantee no fragmentation; only a copying garbage
collector can do that.

Generally speaking, critical operations shouldn't use dynamic
memory. By definition, almost, a dynamic allocation can fail.
Dynamic allocation can be used for non-critical parts of the
system, but you must catch the bad_alloc exception before it
brings the system down.

James Kanze (GABI Software)
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