fork() versus strict virtual memory overcommit handling

September 28, 2012

Unix's fork() is in many ways a wonderful API, but people who want strict virtual memory overcommit handling find it deeply problematic. This is because, as I've written before, fork() is probably the leading way on Unix systems to (theoretically) allocate a lot of memory that you will never use. The more allocated but unused memory you have, the stupider strict overcommit gets (cf); it increasingly denies allocations purely for accounting reasons, instead of from any danger that the system will actually run out of RAM. The corollary is that it's hard to argue that strict overcommit should be the default if systems routinely have significant amounts of allocated but unused memory.

(Why fork() is a good API is another entry. The short version is that fork() is a kernel API, not necessarily a user one.)

It's possible to argue that many instances of unused memory are bad programming practices (or mistakes), and so can at least in theory be discounted when advocating for strict overcommit. This argument is much harder to make with fork(). Straightforward use of fork() followed by exec() can be replaced by APIs like the much more complicated posix_spawn() but there are plenty of other uses of fork() that cannot be (even some uses of fork-then-exec, since posix_spawn() can't do things like change process permissions).

(In the extreme, the arguments of the strict overcommit crowd then boil down to 'well, fork() complicates our life too much so you shouldn't be allowed to use it'. This may sound harsh but it's really what it means to say that historic and natural uses of fork() are now bad practice, at least without a really good reason why.)

PS: vfork() is a hack. Really.

Written on 28 September 2012.
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Last modified: Fri Sep 28 01:12:04 2012
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