[time-nuts] Using the HP 58503a to correct your PC clock

Rick Jones rick.jones2 at hpe.com
Fri Aug 5 17:05:01 EDT 2016

On 08/05/2016 01:41 PM, Attila Kinali wrote:
> Standard network cards will just trigger an IRQ at some point after
> reception and enqueuing of the packet.

Perhaps drifting a bit...

In the broadest of handwaving terms, prior to Gigabit Ethernet, NICs 
(Ethernet anyway) would post an interrupt for each arriving frame.  In 
the 100 Mbit/s Ethernet cards, (and perhaps FDDI and others) they 
started avoiding transmit completion interrupts.  With gigabit Ethernet, 
they started avoiding receive interrupts through interrupt coalescing 
settings.  Different NICs did/do it differently, and some better than 
others.  Under Linux at least, one can use the ethtool utility to tweak 
or even disable interrupt coalescing.  That would likely be Just Fine 
(tm) for a system dedicated to timekeeping, but the performance effects 
on "normal" networking might be more than one desired.

With 10 Gigabit NICs, multiple receive queues come into play (again, 
handwaving a bit).  I would think that some of the later ones would be 
sophisticated enough to enable sending NTP traffic through a specific 
queue/IRQ but I don't know of any 10 GbE NICs with per-queue coalescing 
settings.  My experience with 10 Gbit/s NICs and the likes of say a 
netperf TCP_RR test has been that except perhaps for the worst of them, 
they will not arbitrarily delay a packet arriving at an "idle" time.

What *will* be a non-trivial bummer for such things as a netperf TCP_RR 
test (think ping without "think time") will be power management in the 
processor(s).  Perhaps not visible in a MAN/WAN test setup, but 
certainly visible in a LAN one.

rick jones

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