Re: [V9fs-developer] [PATCH v5 11/11] net/9p: allocate appropriate reduced message buffers

From: Dominique Martinet
Date: Tue Jul 12 2022 - 17:13:49 EST

Dominique Martinet wrote on Wed, Jul 13, 2022 at 04:33:35AM +0900:
> Christian Schoenebeck wrote on Tue, Jul 12, 2022 at 04:31:36PM +0200:
> > So far 'msize' was simply used for all 9p message types, which is far
> > too much and slowed down performance tremendously with large values
> > for user configurable 'msize' option.
> >
> > Let's stop this waste by using the new p9_msg_buf_size() function for
> > allocating more appropriate, smaller buffers according to what is
> > actually sent over the wire.
> >
> > Only exception: RDMA transport is currently excluded from this, as
> > it would not cope with it. [1]

Thinking back on RDMA:
- vs. one or two buffers as discussed in another thread, rdma will still
require two buffers, we post the receive buffer before sending as we
could otherwise be raced (reply from server during the time it'd take to
recycle the send buffer)
In practice the recv buffers should act liks a fifo and we might be able
to post the buffer we're about to send for recv before sending it and it
shouldn't be overwritten until it's sent, but that doesn't look quite good.

- for this particular patch, we can still allocate smaller short buffers
for requests, so we should probably keep tsize to 0.
rsize there really isn't much we can do without a protocol change