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I have a long-living application with frequent memory allocation-deallocation. Will any malloc implementation return freed memory back to the system?
What is, in this respect, the behavior of:
If I have an application whose memory consumption can be very different in daytime and nighttime (e.g.), can I force any of malloc's to return freed memory to the system?
Without such return freed memory will be swapped out and in many times, but such memory contains only garbage.
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When you free() or delete() this memory, it simply gets returned to the heap, not the OS. It's absolutely normal for that memory to not be returned to the operating system until your program exits, as you may request further memory later on.
Yes you absolutely have to use free() after malloc() (as well as closing files and other resources when you're done).
In C, the library function malloc is used to allocate a block of memory on the heap. The program accesses this block of memory via a pointer that malloc returns. When the memory is no longer needed, the pointer is passed to free which deallocates the memory so that it can be used for other purposes.
The malloc line allocates a block of memory of the size specified -- in this case, sizeof(int) bytes (4 bytes). The sizeof command in C returns the size, in bytes, of any type. The code could just as easily have said malloc(4), since sizeof(int) equals 4 bytes on most machines.
The following analysis applies only to glibc (based on the ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system:
mallopt() (defined in malloc.h) does provide an option to set the trim threshold value using one of the parameter option M_TRIM_THRESHOLD, 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 brk() to give back memory to the kernel.
The default value of M_TRIM_THRESHOLD in Linux is set to 128K, setting a smaller value might save space.
The same behavior could be achieved by setting trim threshold value in the environment variable MALLOC_TRIM_THRESHOLD_, with no source changes absolutely.
However, preliminary test programs run using M_TRIM_THRESHOLD 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 brk() tends to be retained.
It is possible to trim the memory arena and give any unused memory back to the system by calling malloc_trim(pad) (defined in malloc.h). This function resizes the data segment, leaving at least pad 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.
The implementation of this modified behavior of free() using malloc_trim could be done using the malloc hook functionality. This would not require any source code changes to the core glibc library.
Using madvise() system call inside the free implementation of glibc.
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