Andi Kleen's blog

Tilting at windmills and other endeavors

Archive for May, 2012

epoll and garbage collection

with 2 comments

epoll, xor lists, pointer compression. What do they have in common? They all don’t play well with automatic garbage collection.

I recently wrote my first program using epoll. And I realized that epoll does not play well with garbage collection. It is one of the few (only?) system calls where the kernel saves a pointer for user space. It supports stuffing other data in there too, like the fd, but for anything non trivial you typically need some state per connection, thus a pointer.

A garbage collector relies on identifying all pointers to an object, so that it can decide whether the object is still used or not.

Now normally user programs should have some other reference to implement time outs and similar. But if they don’t and the only reference is stored in the kernel and the garbage collector comes in at the wrong time the connection object will be garbage collected, even though the connection is still active. This is because the garbage collector cannot see into the kernel.

Something to watch out for.

Written by therapsid

May 1st, 2012 at 1:01 pm

Posted in kernel