Re: update frequency for stats in /proc/net/dev

[Breno Leitao]

On Tue, 2007-12-18 at 07:34 -0500, Mark Seger wrote:
> plotting with gnuplot.  As it turned out, I would very frequently see 
> spikes of 200MB/sec on my 1Gb link.  A colleague noticed the reason was 
> because the network counters were being updated every 0.9765 seconds and 
> this was causing the problem. 
Hello Mark,  when you're measuring performance in small time frame, you
couldn't see packets for a while due the new NAPI scheme. When a device
is using NAPI in a heavy traffic environment, the interrupts are
disabled and the packets are got in a polling manner, through
dev->poll().  
Since dev->poll() is triggered by ksoftirqd, which is a normal lower
priority userspace application, and if you have a heavy load machine,
the packets could be delayed until ksoftirqd is scheduled again.
I am also aware that they backported NAPI to 2.4 kernel, probably
2.4.20, but I am not sure. 

-- 
Breno LeitÃo
Linux eServer IO, LTC Brazil
leitao@xxxxxxxxxxxxxxxxxx

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