Eric Dumazet wrote:
Mark Seger a écrit :
I had mentioned this in my previous post but perhaps it might get
more attention all by itself. I can't say for sure when this
changed, but for the longest time network counters were only updated
once every 0.9765 seconds and unless you used a tools like collectl
that could monitor at fractional intervals, your traffic was
under-reported AND you'd get periodic spikes of double the actual
rate. See http://collectl.sourceforge.net/NetworkStats.html for a
more complete explanation.
Eventually the frequency became better aligned at a 1 second interval
because now the number look better, but the problem I see is that
when the sampling interval is very close to the monitoring interval
you still get periodic incorrect data. Furthermore, you now need to
know which way the counters are updated before you pick a sampling
interval! But the real point is if anyone ever wants to do finer
grained monitoring, say every 1/2 or even tenth of a second, they
can't because the counters won't change between samples. Has this
ever been discussed before?
Yes it was discussed before. Some devices perform counters updates
directly at the NIC level, and one in a while a transfert of counters
is done to the host.
This is supposed to be better, especially on SMP.
Maybe you need to setup accounting rules with iptables, so that you
can perform counter sampling at whatever rate you want ?
Maybe I wasn't clear enough. I'm grabbing the counters from
/proc/net/dev and whatever mechanism is being used only ports them with
a granularity of about once a second. This means any of the standard
tools that use /proc to get their data will all have the same problem.
-mark
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