[201984]

An OS would have a very hard time determining whether a page is "unused" or not. Normal GCs have to know at least which fields of a data structure contain pointers so it can find unreachable objects. To an OS, all memory is just opaque bytes, and it would have no way to know if any given 8 bytes is a pointer to a page or a 64-bit integer that happens to have the same value. This is pretty much why C/C++ don't have garbage collectors currently.

[amelius]

> To an OS, all memory is just opaque bytes, and it would have no way to know if any given 8 bytes is a pointer to a page or a 64-bit integer that happens to have the same value.

This is like saying to an OS all file descriptors are just integers.

[201984]

That's because they are :P

I doubt GC would work on file descriptors either. How could an OS tell when scanning through memory if every 4 bytes is a file descriptor it must keep alive, or an integer that just happens to have the same value?

Not to mention that file descriptors (and pointers!) may not be stored by value. A program might have a set of fds and only store the first one, since it has some way to calculate the others, eg by adding one.

[gpderetta]

A gargbage collector need not be conservative. Interestingly linux (and most posix compliant unices I guess) implements, as last resort, an actual tracing file descriptor garbage collector to track the lifetime of file descriptors: as they can be shared across processes via unix sockets (potentially recursively), arbitrary cycles can be created and reference counting is not enough.