Re: Measuring tools - top and interrupts

On Thu, Jun 22, 2006 at 09:58:08AM -0700, Danial Thom wrote:
--- Erik Mouw <erik@xxxxxxxxxxxxxxxxxxxxx> wrote:
75K packets/s isn't too hard for modern NICs,
especially when using
NAPI.

Well thats just a ridiculous answer, so why
bother?

You polling guys just crack me up. There isn't
much less work to be done with polling. The only
reason you THINK its less work is because the
measuring tools don't work properly. You still
have to process the same number of packets when
you poll, and you have polls instead of
interrupts. Since you can control the # of
interrupts with most cards, there is zero
advantage to polling, and more negatives.

There certainly is less work to be done with polling. Less IRQs means
less expensive context switches, which means a lower system load. See
Documentation/NAPI_HOWTO.txt for information and a link to the Linux
NAPI paper.

And 75K pps may not be "much", but its still at
least 10% of what the system can handle, so it
should measure around a 10% load. 2.4 measures
about 12% load. So the only conclusion is that
load accounting is broken in 2.6.

Network traffic is usually IO bound, not CPU bound. The load figures
top shows tell something about the amount of work the CPU has to do,
not about how busy your PCI bus (or whatever bus the NIC lives on) is.

IIRC the networking layer in 2.6 differs quite a lot from 2.4, so the
load average figures can be quite misleading.


Erik

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| Lab address: Delftechpark 26, 2628 XH, Delft, The Netherlands
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