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copied!<p>The following analysis applies only to glibc (based on ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system:</p> <ol> <li><p><a href="http://www.gnu.org/s/hello/manual/libc/Malloc-Tunable-Parameters.html" rel="noreferrer">mallopt()</a> (defined in <code>malloc.h</code>) does provide an option to set the trim threshold value using one of the parameter option <code>M_TRIM_THRESHOLD</code>, this indicates the minimum amount of free memory (in bytes) allowed at the top of the data segment. If the amount falls below this threshold, glibc invokes <code>brk()</code> to give back memory to the kernel.</p> <p>The default value of <code>M_TRIM_THRESHOLD</code> in Linux is set to 128K, setting a smaller value might save space.</p> <p>The same behavior could be achieved by setting trim threshold value in the environment variable <code>MALLOC_TRIM_THRESHOLD_</code>, with no source changes absolutely.</p> <p>However, preliminary test programs run using <code>M_TRIM_THRESHOLD</code> has shown that even though the memory allocated by malloc does return to the system, the remaining portion of the actual chunk of memory(the arena) initially requested via <code>brk()</code> tends to be retained. </p></li> <li><p>It is possible to trim the memory arena and give any unused memory back to the system by calling <code>malloc_trim(pad)</code> (defined in <code>malloc.h</code>). This function resizes the data segment, leaving at least <code>pad</code> bytes at the end of it and failing if less than one page worth of bytes can be freed. Segment size is always a multiple of one page, which is 4,096 bytes on i386.</p> <p>The implementation for this modified behaviour of <code>free()</code> using <code>malloc_trim</code> could be done using the malloc hook functionality. This would not require any source code changes to the core glibc library.</p></li> <li><p>using <code>madvise()</code> system call inside the free implementation of glibc.</p></li> </ol>

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