Re: [Nbd] strange delay of 5ms

[Wouter Verhelst <w@...112...>]

On Wed, May 04, 2011 at 01:26:02PM +0200, Goswin von Brederlow wrote:
> Wouter Verhelst <w@...112...> writes:
> 
> > On Fri, Apr 22, 2011 at 02:31:23PM +0200, folkert wrote:
> >> Hi,
> >> 
> >> I noticed something odd:
> >> 
> >> for(;;) {
> >> 
> >> 	// get timestamp in ms start
> >> 
> >> 	read(fd, size_of_nbd_request)
> >> 
> >> 	// get timestamp in ms end
> >> 
> >> 	printf("delay: %f\n", end - start);
> >> 
> >> 	// process request
> >> }
> >> 
> >> Then, on a localhost-connection with:
> >> dd if=/dev/zero of=/dev/nbd0
> >> 
> >> in this setup I noticed that it takes 5ms for the request header to come
> >> in, but only when the processing takes a while.
> >> What can be the cause of this?
> >
> > I'm not sure. I'm aware that nbd-server performance isn't exactly
> > perfect, and I plan on working on that, but it would involve some
> > significant work.
> 
> Could this be due to tcp corking? I.e. the kernel waiting for more data
> to send in a single frame?

Not likely, since nbd-server doesn't set TCP_CORK. But of course it's not
impossible either

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