Messages in this thread | | | Date | Sun, 03 Feb 2008 22:53:26 -0800 | From | Max Krasnyansky <> | Subject | Re: [CPUISOL] CPU isolation extensions |
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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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