Re: CLOCK_MONOTONIC datagram timestamps by the kernel

From: John
Date: Fri Mar 02 2007 - 04:46:50 EST


Eric Dumazet wrote:

John wrote:

Consider an idle Linux 2.6.20-rt8 system, equipped with a single PCI-E
gigabit Ethernet NIC, running on a modern CPU (e.g. Core 2 Duo E6700).
All this system does is time stamp 1000 packets per second.

Are you claiming that this platform *cannot* handle most packets within
less than 1 microsecond of their arrival?

Yes I claim it. You expect too much of this platform, unless "most" means
10 % for you ;)

By "most" I meant more than 50%.

Has someone tried to measure interrupt latency in Linux? I'd like to plot the distribution of network IRQ to interrupt handler latencies.

If you replace "1 us" by "50 us", then yes, it probably can do it, if "most" means 99%, (not 99.999 %)

I think we need cold, hard numbers at this point :-)

Anyway, if you want to play, you can apply this patch on top of linux-2.6.21-rc2 (nanosecond resolution infrastructure needs 2.6.21)
I let you do the adjustments for rt kernel.

Why does it require 2.6.21?

This patch converts sk_buff timestamp to use new nanosecond infra
(added in 2.6.21)

Is this mentioned somewhere in the 2.6.21-rc1 ChangeLog?
http://kernel.org/pub/linux/kernel/v2.6/testing/ChangeLog-2.6.21-rc1

Regards.
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