[ntp:questions] NTP vs RADclock?

Kasper Pedersen ntp at taur.dk
Mon Jun 11 07:39:36 UTC 2012

On 06/11/2012 02:29 AM, unruh wrote:

> Anyway, from the very preliminary tests, this seems to have solved the
> main problem. Of course that problem that on that one machine about half
> the packets have a .14ms return time and the other half have 1.5ms
> return time is not solved. From the scatter plot, this delay is occuring
> on the return leg of the round trip. Something is delaying them by a ms.
> on that return trip. The ethernet card on this machine is that Intel
> Corporation 82557/8/9 Ethernet Pro 100 (rev 08)

That figure (1.5ms) makes me recall a patch I did for a machine with
an old 82559 rev 8:

"Linux: Tweak coalescing in e100 aka Intel Pro/100. i82559(08,09) and
possibly also i82551(0F,10) will randomly delay received packets up to
1.5 ms. This patch disables coalescing for packets less than 256 bytes
so that NTP traffic will not suffer. "
It changes a microcode constant so that ntp-sized packets also bypass
coalescing, not just ACK-sized packets.


You may also want (not mine)
particularly if the machine is not running tickless.

ARP will make it appear as if there is an occasional ~500us delay in the
outbound direction. When running non-tickless, there may be an additional
one-tick (1ms typical) delay in sending the ARP request.
The easy test fot this is to put in a static ARP entry on each end.

(assuming you, or a friend, won't mind building your own kernel)

/Kasper Pedersen

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