Re: Thread implementations...
Richard Gooch (Richard.Gooch@atnf.CSIRO.AU)
Sun, 21 Jun 1998 13:32:59 +1000
Dean Gaudet writes:
> On Sun, 21 Jun 1998, Richard Gooch wrote:
> > People seem to be very interested in the new connection case. This
> > doesn't seem all that exiting or interesting to me (just have one
> > thread blocking in accept(2) and store the new FD in a global
> > array). To me the problem of processing data on existing connections
> > is more interesting (and harder to solve: hence more interesting:-).
> > Is there something deep I'm missing here?
> The new connection case is actually pretty much the same as all the other
> cases, but maybe just easier to explain.
> Suppose you do what you suggest. Have a single accept() thread which
> plops FDs into a global queue. It also presumably tweaks a condition
> variable to awake a waiting processing thread. To start processing a new
> socket there are two context switches, one into the accept thread, and one
> into a processing thread.
> That second switch is a waste. Instead you could mutex protect accept()
> and go into it with the processing thread, and release the mutex on the
> way out. Then you have only one context switch for each new socket. This,
> incidentally, is almost what happens inside the kernel... except the
> kernel uses wake-all semantics (freebsd seems to have solved this for
> accept... alan and linus say there are difficulties in solving it, so it
> hasn't been solved in linux yet). So you can actually drop the mutex.
> Back to the single thread/accept queue. There's only a single thread in
> accept(), and if the box you're running on has two processors you're not
> exploiting the parallelism available. You could do some fun gymnastics at
> the user level to put multiple threads waiting on accept() ... but that's
> overkill because usually the kernel is the best judge of the parallelism
> available. So just putting a collection of threads into accept() and
> letting the kernel sort it out solves this.
> But does it? Now you have to figure out how many threads you should have
> waiting in accept at any one time. (In Apache, this is the joyful
> nonsense of deciding the right MinSpareServers and MaxSpareServers
> settings to handle load spikes and parallelism and all that fun stuff.)
> And your threads waiting in accept are kernel scheduled resources
> consuming kernel ram.
> If all your program did was call accept() you'd be able to figure this all
> out pretty easily. But presumably you do more than that.
> accept() is interesting because it is actually an event queue... it's a
> queue of new connections arriving on a single socket. The kernel has all
> the knowledge it needs to multiplex the socket connections in a way
> suitable to the hardware.
> But accept() is limiting because it only handles a single listening
> socket. If your web server has both port 80 and port 443, you need some
> way to accept connections on both. You prefer to run a single web server
> to take advantage of shared configuration memory and other resources. Now
> you need some method of accepting connections on multiple sockets. You
> could just implement two pools of threads, one for each socket. But that
> doesn't scale to many many sockets (which some people actually do use, for
> better or for worse) ... and now you have to tune min/maxspare parameters
> for multiple pools, what a headache.
> What you'd really like is a way to say "accept a connection on any of
> these sockets" so that you can continue to maintain a single pool of
> threads. The single pool is not only easier to configure, it has the
> benefits of cache locality. Presumably everything in the pool is
> identical -- all the threads are capable of handling the returned socket.
> The kernel can use LIFO on the waiting threads because the last-in thread
> is most likely to still have data in L1.
> But really the same can be said for read/write as well as accept. Suppose
> you had a hybrid user/kernel threads package which uses co-operative
> pre-emption, i/o points are pre-emption points. When a user-thread does
> an i/o the package just notes that the user-thread is blocked. Then it
> asks the kernel "give me an FD which is ready for I/O". It determines which
> user-thread is waiting for that FD and dispatches that user-thread. In
> this model you need as many kernel threads as there is parallelism to
> exploit. The user-threads are written in the standard procedural manner,
> which is easy to program (rather than the stateful manner of something
> like squid where all the state transitions for i/o state are explicit
> and in the control of the programmer).
OK, I agree that the accept case is a specific example of checking a
large number of FDs for some activity, be it read, write or new
> Central to that is the "give me an FD which is ready for I/O" step. This
> is where select/poll are traditionally used... but the question is really
> a wake-one question, and select/poll are wake-all primitives. The
> kernel-threads in this example are all equivalent, any one of them can
> switch to any of the user-threads. Can you see how read/write are
> pretty similar to accept and how all the problems are related?
Yep. I was thinking single-socket, multiple-connections.
Multiple-socket and multiple-connections is the general case. I still
don't agree that we *have* to have event completion ports, though. See
previous message about a simple (IMHO) userspace solution.
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