linux-kernel - Re: [CPUISOL] CPU isolation extensions

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Date:	Sun, 03 Feb 2008 22:53:26 -0800
From:	Max Krasnyansky <maxk@...lcomm.com>
To:	Daniel Walker <dwalker@...sta.com>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <srostedt@...hat.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Paul Jackson <pj@....com>
Subject: Re: [CPUISOL] CPU isolation extensions

Hi Daniel,

Sorry for not replying right away.

Daniel Walker wrote:
> On Mon, 2008-01-28 at 16:12 -0800, Max Krasnyanskiy wrote:
> 
>> Not accurate enough and way too much overhead for what I need. I know at this point it probably 
>> sounds like I'm talking BS :). I wish I've released the engine and examples by now. Anyway let 
>> me just say that SW MAC has crazy tight deadlines with lots of small tasks. Using nanosleep() & 
>> gettimeofday() is simply not practical. So it's all TSC based with clever time sync logic between
>> HW and SW.
> 
> I don't know if it's BS or not, you clearly fixed your own problem which
> is good .. Although when you say "RT patches cannot achieve what I
> needed. Even RTAI/Xenomai can't do that." , and HRT is "Not accurate
> enough and way too much overhead" .. Given the hardware your using,
> that's all difficult to believe.. You also said this code has been
> running on production systems for two year, which means it's at least
> two years old .. There's been some good sized leaps in real time linux
> in the past two years ..

I've been actually tracking RT patches fairly closely. I can't say I tried all of them but I do try
them from time to time. I just got latest 2.6.24-rt1 running on HP xw9300. Looks like it does not handle
CPU hotplug very well, I manged to kill it by bringing cpu 1 off-line. So I cannot run any tests right 
now will run some tomorrow.

For now let me mention that I have a simple tests that sleeps for a millisecond, then does some bitbanging 
for 200 usec. It measures jitter caused by the periodic scheduler tick, IPIs and other kernel activities.
With high-res timers disabled on most of the machines I mentioned before it shows around 1-1.2usec worst case. 
With high-res timers enabled it shows 5-6usec. This is with 2.6.24 running on an isolated CPU. Forget about
using a user-space timer (nanosleep(), etc). Even scheduler tick itself is fairly heavy.
gettimeofday() call on that machine takes on average 2-3usec (not a vsyscall) and SW MAC is all about precise 
timing. That's why I said that it's not practical to use that stuff for me. I do not see anything in -rt kernel 
that would improve this.

This is btw not to say that -rt kernel is not useful for my app in general. We have a bunch of soft-RT threads
that talk to the MAC thread. Those would definitely benefit. I think cpu isolation + -rt would work beautifully
for wireless basestations.

Max 
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