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From: Max Krasnyansky <>
To: Daniel Walker <>
Cc: Peter Zijlstra <>,, Ingo Molnar <>,
	Steven Rostedt <>,
	Gregory Haskins <>, Paul Jackson <>
Subject: Re: [CPUISOL] CPU isolation extensions
Date: Sun, 03 Feb 2008 22:53:26 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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.


  reply	other threads:[~2008-02-04  6:55 UTC|newest]

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-28  4:09 maxk
2008-01-28  4:09 ` [PATCH] [CPUISOL] Add config options for CPU isolation maxk
2008-01-28  4:09   ` [PATCH] [CPUISOL] Export CPU isolation bits maxk
2008-01-28  4:09     ` [PATCH] [CPUISOL] Do not route IRQs to the CPUs isolated at boot maxk
2008-01-28  4:09       ` [PATCH] [CPUISOL] Support for workqueue isolation maxk
2008-01-28  4:09         ` [PATCH] [CPUISOL] Isolated CPUs should be ignored by the "stop machine" maxk
2008-01-28  9:08 ` [CPUISOL] CPU isolation extensions Peter Zijlstra
2008-01-28 14:59   ` Paul Jackson
2008-01-28 16:34     ` Steven Rostedt
2008-01-28 16:44       ` Peter Zijlstra
2008-01-28 18:54         ` Max Krasnyanskiy
2008-01-28 18:46       ` Max Krasnyanskiy
2008-01-28 19:00         ` Steven Rostedt
2008-01-28 20:22           ` Peter Zijlstra
2008-01-28 21:42             ` Max Krasnyanskiy
2008-02-05  0:32             ` CPU isolation and workqueues [was Re: [CPUISOL] CPU isolation extensions] Max Krasnyanskiy
2008-01-28 18:37     ` [CPUISOL] CPU isolation extensions Max Krasnyanskiy
2008-01-28 19:06       ` Paul Jackson
2008-01-28 21:47         ` Max Krasnyanskiy
2008-01-31 19:06         ` Integrating cpusets and cpu isolation [was Re: [CPUISOL] CPU isolation extensions] Max Krasnyanskiy
2008-02-02  6:16           ` Paul Jackson
2008-02-03  5:57             ` Max Krasnyansky
2008-02-03  7:53               ` Paul Jackson
2008-02-04  6:03                 ` Max Krasnyansky
2008-02-04 10:54                   ` Paul Jackson
2008-02-04 23:19                     ` Max Krasnyanskiy
2008-02-05  2:46                       ` Paul Jackson
2008-02-05  4:08                         ` Max Krasnyansky
2008-01-28 18:32   ` [CPUISOL] CPU isolation extensions Max Krasnyanskiy
2008-01-28 19:10     ` Paul Jackson
2008-01-28 23:41     ` Daniel Walker
2008-01-29  0:12       ` Max Krasnyanskiy
2008-01-29  1:33         ` Daniel Walker
2008-02-04  6:53           ` Max Krasnyansky [this message]
2008-01-31 12:16 ` Mark Hounschell
2008-01-31 19:13   ` Max Krasnyanskiy

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