On Jul 25, 2007, at 22:03:37, [email protected] wrote:
On Wed, 25 Jul 2007, Satyam Sharma wrote:
On 7/25/07, Lars Ellenberg <[email protected]> wrote:
On Wed, Jul 25, 2007 at 04:41:53AM +0530, Satyam Sharma wrote:
[...]
But where does the "send" come into the picture over here -- a
send won't block forever, so I don't foresee any issues
whatsoever w.r.t. kthreads conversion for that. [ BTW I hope
you're *not* using any signals-based interface for your kernel
thread _at all_. Kthreads disallow (ignore) all signals by
default, as they should, and you really shouldn't need to write
any logic to handle or > do-certain-things-on-seeing a signal
in a well designed kernel thread. ] and the sending latency is
crucial to performance, while the recv will not timeout for the
next few seconds. Again, I don't see what sending latency has
to do with a kernel_thread to kthread conversion. Or with
signals, for that matter. Anyway, as Kyle Moffett mentioned
elsewhere, you could probably look at other examples (say
cifs_demultiplexer_thread() in fs/cifs/connect.c).
the basic problem, and what we use signals for, is: it is
waiting in recv, waiting for the peer to say something. but I
want it to stop recv, and go send something "right now".
That's ... weird. Most (all?) communication between any two
parties would follow a protocol where someone recv's stuff, does
something with it, and sends it back ... what would you send
"right now" if you didn't receive anything?
becouse even though you didn't receive anything you now have
something important to send.
remember that both sides can be sitting in receive mode. this puts
them both in a position to respond to the other if the other has
something to say.
Why not just have 2 threads, one for "sending" and one for
"receiving". When your receiving thread gets data it takes
appropriate locks and processes it, then releases the locks and goes
back to waiting for packets. Your sending thread would take
appropriate locks, generate data to send, release locks, and transmit
packets. You don't have to interrupt the receive thread to send
packets, so where's the latency problem, exactly?
If I were writing that in userspace I would have:
(A) The pool of IO-generating threads (IE: What would ordinarily be
userspace)
(B) One or a small number of data-reception threads.
(C) One or a small number of data-transmission threads.
When you get packets to process in your network-reception thread(s),
you queue appropriate disk IOs and any appropriate responses with
your transmission thread(s). You can basically just sit in a loop on
tcp_recvmsg=>demultiplex=>do-stuff. When your IO-generators actually
make stuff to send you queue such data for disk IO, then packetize it
and hand it off to your data-transmission threads.
If you made all your sockets and inter-thread pipes nonblocking then
in userspace you would just epoll_wait() on the sockets and pipes and
be easily able to react to any IO from anywhere.
In kernel space there are similar nonblocking interfaces, although it
would probably be easier just to use a couple threads.
Cheers,
Kyle Moffett
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