Re: [take25 1/6] kevent: Description.

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Ulrich Drepper wrote:
Evgeniy Polyakov wrote:
+ int kevent_commit(int ctl_fd, unsigned int start, + unsigned int num, unsigned int over);

I think we can simplify this interface:

   int kevent_commit(int ctl_fd, unsigned int new_tail,
                     unsigned int over);

The kernel sets the ring_uidx value to the 'new_tail' value if the tail pointer would be incremented (module wrap around) and is not higher then the current front pointer. The test will be a bit complicated but not more so than what the current code has to do to check for mistakes.

This approach has the advantage that the commit calls don't have to be synchronized. If one thread sets the tail pointer to, say, 10 and another to 12, then it does not matter whether the first thread is delayed. If it will eventually be executed the result is simply a no-op and since second thread's action supersedes it.

Maybe the current form is even impossible to use with explicit locking at userlevel. What if one thread, which is about to call kevent_commit, if indefinitely delayed. Then this commit request's value is never taken into account and the tail pointer is always short of what it should be.

I'm really wondering is designing for N-threads-to-1-ring is the wisest choice?

Considering current designs, it seems more likely that a single thread polls for socket activity, then dispatches work. How often do you really see in userland multiple threads polling the same set of fds, then fighting to decide who will handle raised events?

More likely, you will see "prefork" (start N threads, each with its own ring) or a worker pool (single thread receives events, then dispatches to multiple threads for execution) or even one-thread-per-fd (single thread receives events, then starts new thread for handling).

If you have multiple threads accessing the same ring -- a poor design choice -- I would think the burden should be on the application, to provide proper synchronization.

If the desire is to have the kernel distributes events directly to multiple threads, then the app should dup(2) the fd to be watched, and create a ring buffer for each separate thread.

	Jeff


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