Subject: Re: CLOCK_MONOTONIC datagram timestamps by the kernel

From: John <[email-masked]>
Date: Fri, 02 Mar 2007 10:26:57 +0100

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.