David Larochelle writes: > > Are languages that have mark and sweep garbage collection better about > returning memory to the system than languages like Perl that use reference > count garbage collection.
Not in practice. While that potential is there, in it is not utilized. Most programs that allocate & then free a lot of memory will either terminate in the near future or will reallocate that memory again in the near future. One can imagine two programs on a system which took turns allocating & then freeing a lot of memory, such that it would make sense for each of them to return the memory to the host for it to allocate to the other -- but since such programs are rare, the chances of having two of them that happen to synchronize thusly is even lower -- it's just not worth the effort to make it happen -- particularly because there's another approach which makes more sense and permits application to exercise fine control over memory use. Programs that want to do this kind of thing -- alone or cooperatively -- should use mmap (UNIX) or CreateFileMapping (Windows). This provides the application with a virtual address mapping that is backed (generally) by a file on disk. When the application wants to release that memory, it merely unmaps the file. At the point where actual content is read or written to the file, physical RAM pages are allocated & used, but they are released with the file itself. This is generally the technique programs should use when they want control over this kind of thing -- mmap also makes it easy for one program to map-in what another program has mapped-out, thus instantly sharing a complex data structure (using offsets rather than absolute addressing for internal pointers). If two programs both mmap a file at the same time, they can share the physical memory and thus communicate with each other that way (generally also using semaphores for locking). There are a number of options which exist other than simply mapping a regular file -- it is also possible to map directly against swap, and to control how the mapped pages are addressed and cached and so forth. This is essentially a user-space mechanism to tell the kernel exactly how certain segments of memory need to be handled. _______________________________________________ Boston-pm mailing list Boston-pm@mail.pm.org http://mail.pm.org/mailman/listinfo/boston-pm
