> On Thu, 13 Jul 2000, Stephen McKay wrote:
> >>Guess it will show up if you measure latencies (or your application is
> >>doing lots of RPCs). But as soon as there is a cheap 100baseT switch in
> >>the path to the destination, there will be store-and-forward at work ;-)
> >
> >Does anyone here actually measure these latencies?  I know for a fact
> >that nothing I've ever done would or could be affected by extra latencies
> >that are as small as the ones we are discussing.  Does anybody at all
> >depend on the start-transmitting-before-DMA-completed feature we are
> >discussing?
> I don't like the idea of removing that feature.  Perhaps it should be a
> sysctl or ifconfig option, but it should definitely remain available.
> Those minute latencies are critical to those of us who use MPI for
> complex parallel calculations.

I have to agree here.  The store and forward adds an approximate
11uS (by theory under ideal conditions 1500bytes@132MB/s = 11uS,
practice actually makes this worse as typical PCI does something
less than 100MB/s or 15uS) to a 120uS packet time on the wire (again,
ideal, but here given that switches, and infact often cut-through
switches, are used for these types of things, ideal and practice
are very close.)

I don't think these folks, nor myself, are wanting^H^H^H^H^H^H^Hilling
to give up 12.5%.

Rod Grimes - KD7CAX @ CN85sl - (RWG25)               [EMAIL PROTECTED]

To Unsubscribe: send mail to [EMAIL PROTECTED]
with "unsubscribe freebsd-current" in the body of the message

Reply via email to