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Date: Mon, 8 Jul 2002 08:27:56 -0400 (EDT)
From: "Richard B. Johnson" <root@chaos.analogic.com>
Reply-To: root@chaos.analogic.com
To: Xinwen - Fu <xinwenfu@cs.tamu.edu>
cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: timer queue is still influenced by network load
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On Wed, 3 Jul 2002, Xinwen - Fu wrote:

> Richard,
> 	I did a few experiments using the example (jiq, I changed jiffies 
> to do_gettimeofday() ) from Linux Device
> Driver, 2nd version (p196). 
> 
> 	I have two machines m1 and m2. On m1, I run a timer queue (jiq)
> module. Then I download a big file from m1 to m2. The timings are
> different between before ftp and during ftp.
> 

> 
> 
> It shows that
> timer queue is still not accurate. So
> the conclusion of " you're guaranteed that the queue will run at the next
> clock tick, thus eliminating latency caused by system load" is WRONG!!!
> 
> 	What is your opinion?

Well you are guaranteed that it will run. You just don't know how fast
it will run. The bottom-half code run off the timer-queue is run with
the interrupts enabled. It can get interrupted and it may be interrupted
by network driver code that loops in ISRs, taking a large percentage
of the CPU cycles.

So, I don't think you are measuring what you think you are measuring.


Cheers,
Dick Johnson

Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).

                 Windows-2000/Professional isn't.

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