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SubjectRe: kqueue microbenchmark results
On Wed, Oct 25, 2000 at 11:40:28AM -0700, Simon Kirby wrote:
> On Wed, Oct 25, 2000 at 12:23:07PM -0500, Jonathan Lemon wrote:
> > Consider a program which reads from point A, writes to point B. If
> > the buffer associated with B fills up, then we don't want to continue
> > reading from A.
> >
> > A/B may be network sockets, pipes, or ptys.
> Fine, but we can bind the event watching to the device or socket or pipe
> that will clog up, right? In which case, we'll later get a write event
> (just like with select()), and then once there is some progress you can
> go back to read()ing from the original descriptor. This is even easier
> than using select() because you don't have to take the descriptor out of
> the read set and put it in the write set temporarily -- it will
> automatically work that way.

Yes, but with the above, you can't use get_event() as your main
dispatching loop to do the read() call any more, since there may
be no notifications pending in the queue. So you have to expand
your main loop to include both get_event() as well as walk the
"these descriptors may have partial data" list.

Also, as Jamie pointed out, with kqueue/select you can do:


while with a pure edge-triggered scheme, you either must do:

bind_event/read/.../read == 0/write

Or maintain your own "this descriptor may have data" list.

Also, consider the following scenario for the proposed get_event():

1. packet arrives, queues an event.
2. user retrieves event.
3. second packet arrives, queues event again.
4. user reads() all data.

Now, next time around the loop, we get a notification for an event
when there is no data to read. The application now must be prepared
to handle this case (meaning no blocking read() calls can be used).

Also, what happens if the user closes the socket after step 4 above?

The user now receives a notification for a fd which no longer exists,
or possibly has been reused for another connection. This may or may
not make a difference to the application, but it must be prepared to
handle it anyway. I believe that Zack Brown ran into this problem with
one of the webservers he was writing.

> > You can find my paper at
> I'll go and read it now. :)

The paper talks about some of the issues we have been discussing, as
well as the design rationale behind kqueue. I'd be happy to answer
any questions about the paper.
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