netdev - Re: Using jiffies for tcp_time

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Date:	Fri, 25 May 2012 18:23:40 +0200
From:	Srećko Jurić-Kavelj 
	<srecko.juric-kavelj@....hr>
To:	Dave Taht <dave.taht@...il.com>
Cc:	Chris Friesen <chris.friesen@...band.com>, netdev@...r.kernel.org
Subject: Re: Using jiffies for tcp_time_stamp?

On Fri, May 25, 2012 at 6:17 PM, Dave Taht <dave.taht@...il.com> wrote:
> On Fri, May 25, 2012 at 4:58 PM, Chris Friesen
> <chris.friesen@...band.com> wrote:
>> I don't know if it would make any difference to the tcp algorithms, but
>> certainly on some architectures you can get a fast and accurate hardware
>> timestamp.
>
> I would be interested in someone doing that experiment in light of the
> codel work.

I've looked this up in other implementations, e.g. FreeBSD uses 1ms
granularity no matter what HZ says, NetBSD has 500ms ticks, ...

I guess that granularity also depends on the retransmit timers used. I
didn't make out what's the precision of the timers that Linux uses in
TCP, but I guess it uses high resolution timers? At least on x86?

I've done a simple experiment by repeatedly calling clock_gettime
(from userspace, but I guess it ends up as a vsyscall). I get >17
million calls per second on a Q6600.

--
JKS
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