Garbage Collection in C Programs

LISP and Java programmers take garbage collection for granted. With the Boehm-Demers-Weiser library, you easily can use it in C and C++ projects, too.


gctest -gu -s 128 -n 150000 -l 5

Under these conditions, the improvement of GC over malloc/free was around 20%, 0.85 vs. 0.60 seconds. Recall, though, the GC library always clears the allocated blocks, but malloc() does not. The overhead associated with this operation grows linearly with node size and thus is more important with larger nodes. To make a fair comparison, then, we need to substitute malloc() with calloc() at those points where GC_malloc() is called, as happens in:

gctest -tuc -s 128 -n 150000 -l 5

This test yields an execution time of 0.88 seconds and brings the GC improvement to 32%. Heap expansion is greater in the GC case, with a value of 1.7 vs. 1.0. Allocation latency is practically the same for both traditional and GC management, although a larger latency variation was experienced in the latter. Enabling incremental collection (-i option) did not lower the variation, although introducing calls to GC_free() (-F option) to explicitly free the list nodes explicitly actually did yield better results than the malloc/free case, on both execution time and latency. However, in this case we are not strictly using a real garbage collection approach.

Testing even larger memory blocks makes the difference between traditional and GC memory management quite noticeable. On 4KB nodes, GC performed quite poorly in comparison with malloc/free on execution time, 0.85 vs. 2 seconds; heap expansion, 2.75 vs. 1; and latency, 0.7 milliseconds vs. 1.6 microseconds. When compared with calloc/free performance, the execution time of GC still is quite competitive (40% faster). But, issues related to heap and latency remain.


GC techniques often are surrounded by myths and legends. In this article we have shown that GC actually can perform better than malloc/free. These advantages do not come for free, however, and the correct use of the library requires minimum knowledge on its internal mechanisms.

There is no final verdict on the suitability of GC for C programs. For now, the BDW library can be one more tool in your box, to be given serious consideration the next time you deal with a complex piece of software. Several open-source projects, like the Mono Project and GNU gcj Java runtime, have been using it for a while.

Finally, the BDW library also can be used proficiently as a leak detector; the Mozilla team uses it this way, for example. For more information, have a look at the included documentation.


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