[ntp:questions] Sub-millisecond NTP synchronization for local network

Terje Mathisen terje.mathisen at hda.hydro.com
Tue Dec 9 11:57:14 UTC 2008


Rick Jones wrote:
> Hal Murray <hal-usenet at ip-64-139-1-69.sjc.megapath.net> wrote:
>> Some Ethernet adapters have a bug/feature similar to RS-232 chips.  The
>> idea is to batch interrupts to reduce overhead.  Ethernets do it by
>> only making one interrupt for several packets as compared to several
>> bytes for the RS-232 chips.
> 
>> I'd expect gigabit cards/chips will be more likely to have that
>> feature than old "slow" 100 megabit chips.
> 
> Virtually all the GbE NICs I've ever encountered have such a feature.
> And literally all the 10 GbE NCIs I've encountered have it.
> 
> You can see such behaviour in a NIC/driver which does it "badly" (IMO)
> with a netperf TCP_RR test.  For example if you only ever see
> 8000-12000 single-byte transactions per second with a "contemporary"
> system.
> 
> A while back I did a writeup on the tradeoff the NIC/driver strappings
> were making.  A version of it can be found at:
> 
> ftp://ftp.cup.hp.com/dist/networking/briefs/nic_latency_vs_tput.txt
> 

Nice work!

Terje

-- 
- <Terje.Mathisen at hda.hydro.com>
"almost all programming can be viewed as an exercise in caching"




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