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Re: [Qemu-devel] [RFC 3/5] nbd: Use aio_set_fd_handler2()
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [RFC 3/5] nbd: Use aio_set_fd_handler2() |
Date: |
Thu, 5 Jun 2014 10:12:23 +0200 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Wed, Jun 04, 2014 at 08:02:06PM +0200, Paolo Bonzini wrote:
> Il 04/06/2014 14:37, Stefan Hajnoczi ha scritto:
> >Why is this design cleaner? Because NBD code doesn't have to worry
> >about fd handlers. It uses straightforward coroutine send/recv for
> >socket I/O inside nbd_read_req() and nbd_write_resp(). It's easy to see
> >that only one coroutine receives from the socket and that only one
> >coroutine writes to the socket.
>
> I don't understand how this would work without managing fd handlers.
fd handlers still need to be managed, but not by NBD code. They must be
managed by coroutine recv/send utility functions. In other words, fd
handlers are used locally, not globally.
def co_recv(fd, buf):
while True:
nbytes = recv(fd, buf, len(buf))
if nbytes == -1:
if errno == EINTR:
continue
if errno == EAGAIN or errno == EWOULDBLOCK:
aio_set_fd_read_handler(fd, co_recv_cb)
qemu_coroutine_yield()
aio_set_fd_read_handler(fd, NULL)
continue
return nbytes
The send function is similar.
This does require an extension to the AioContext API. We need to be
able to modify the read/write callback independently without clobbering
the other callback. This way full duplex I/O is possible.
> - If you don't want to receive anything (because you reached the maximum
> queue depth), and the client sends something, the read handler will busy
> wait. The current code solves it with can_read; you could do it by enabling
> or disabling the read handler as you need, but the idea is the same.
>
> - If you're not sending anything, a socket that has a write handler will
> have POLLOUT continuously signaled and again you'd busy wait. Since there
> is no can_write, nbd.c instead enables/disables the write handler around
> nbd_co_send_reply.
You only install an fd handler when you want to read/write. This does
mean that the request coroutine needs to be woken up by the response
coroutine if we were at maximum queue depth.
Stefan