Re: [Qemu-devel] [PATCH 0/3] nbd: use TCP_NODELAY

[Nicholas Thomas <nick@bytemark.co.uk>]

On Mon, 2013-04-15 at 16:14 +0200, Stefan Hajnoczi wrote:
> The nbd block driver should use TCP_NODELAY.  Nick Thomas <nick@bytemark.co.uk>
> measured a 40 millisecond latency added by the Naggle algorithm.
> 
> This series turns on TCP_NODELAY.  This requires that we use TCP_CORK to
> efficiently send NBD requests that contain a payload after the header.
> 
> Finally, fix a bug where we forget to unlock a mutex when sending fails.
> 
> Stefan Hajnoczi (3):
>   nbd: unlock mutex in nbd_co_send_request() error path
>   nbd: use TCP_CORK in nbd_co_send_request()
>   nbd: set TCP_NODELAY
> 
>  block/nbd.c | 25 +++++++++++++++++++------
>  1 file changed, 19 insertions(+), 6 deletions(-)
> 

Tested-by: Nick Thomas <nick@bytemark.co.uk>

Running "dd if=/dev/zero of=/dev/hda bs=4096 count=1" - latency is
reduced from ~40ms to ~0.2ms. The entire NBD write request ends up in a
single ethernet frame, where mtu supports it (verified with wireshark).

Thanks!

/Nick