latency & real-time-ness.
I have been doing some tests with 2.4.19-pre2-ac2 with regard to network latency. When running a steady stream of 138byte UDP packets at 115 packets per second, I see about .1% of the packets take more than 5 miliseconds to go from user-space to user-space on a 1Ghz PIII machine.
At 50Mbps (bi directional), I see a much wider latency spread, with some packets taking up to 300ms or higher to get from A to B. The CPU load ranges from about 30% to 80% utilization at this speed...
I'm running the program at nice -18.
So, what kind of things can I do to decrease the latency?
Would the low-latency patch help me?
Are there any scheduling tricks I can use to tell the kernel that my program should get to run as soon as it wants to?
Thanks,
Ben
Signature · 3 lines
Ben Greear <[email protected]> <Ben_Greear AT excite.com> President of Candela Technologies Inc http://www.candelatech.com ScryMUD: http://scry.wanfear.com http://scry.wanfear.com/~greear
Quoted 2 lines
I'm running the program at nice -18.
So, what kind of things can I do to decrease the latency?
Hack up the ksoftirq stuff to only fall back to ksoftirqd after about 500 iterations instead of one is one little detail to deal with
Quoted 1 line
Would the low-latency patch help me?
Yes
Quoted 6 lines · Alan Cox
Quoted 3 lines
I'm running the program at nice -18.
So, what kind of things can I do to decrease the latency?Hack up the ksoftirq stuff to only fall back to ksoftirqd after about 500 iterations instead of one is one little detail to deal with
Can someone expound slightly on this?
Quoted 6 lines
Quoted 4 lines
Would the low-latency patch help me?
Yes
Excellent, I'm compiling that now....
Thanks,
Ben
Signature · 3 lines
Ben Greear <[email protected]> <Ben_Greear AT excite.com> President of Candela Technologies Inc http://www.candelatech.com ScryMUD: http://scry.wanfear.com http://scry.wanfear.com/~greear
Quoted 4 lines · Alan Cox
Quoted 3 lines
I'm running the program at nice -18.
So, what kind of things can I do to decrease the latency?
Quoted 8 lines
Hack up the ksoftirq stuff to only fall back to ksoftirqd after about 500 iterations instead of one is one little detail to deal with
Quoted 2 lines
Would the low-latency patch help me?
Yes
It might be very difficult to fix up the low latency patch for the latest -ac, but the mini-low-latency patch should go right in - that should fix the worst of it, and I've run 2.4.19-pre2-ac1 with the mini low-latency patch.
It's part of the -aa patch collection, just look for "00_lowlatency-fixes-4" or so.
Joe
Ben Greear wrote
Quoted 7 lines
Quoted 3 lines
Would the low-latency patch help me?
Yes
Excellent, I'm compiling that now....
Eh?
The full-on low latency patch from Andrew Morton?
You might want to make some diffs available
since AFIK that would have involved quite a bit
of hand editing to fix rejects...
Joe
Quoted 2 lines
It might be very difficult to fix up the
low latency patch for the latest -ac,
You should be able to just dump out the vm part of it - Rik put that into rmap anyuway afaik
Quoted 7 lines · Alan Cox
Quoted 3 lines
It might be very difficult to fix up the
low latency patch for the latest -ac,You should be able to just dump out the vm part of it - Rik put that into rmap anyuway afaik
Ah, excellent - good to know, I'll check that out tonight -
BTW 2.4.19-pre2-ac1 was pretty good for me - The GUI remained snappy and the mp3s played smoothly all while running dbench 128 - The O(1) scheduler and rmap stuff look like a win -
Joe
Quoted 5 lines · J Sloan
Ben Greear wrote
Quoted 3 lines
Quoted 3 lines
Would the low-latency patch help me?
Yes
Quoted 8 lines
Quoted 1 line
Excellent, I'm compiling that now....
Eh?
The full-on low latency patch from Andrew Morton?
You might want to make some diffs available
since AFIK that would have involved quite a bit
of hand editing to fix rejects...
I found this patch:
preempt-kernel-rml-2.4.19-pre2-ac2-1.patch
It applied cleanly...looks like maybe this isn't
the low-latency patch though now that I look at
it a little closer.
Signature · 3 lines
Ben Greear <[email protected]> <Ben_Greear AT excite.com> President of Candela Technologies Inc http://www.candelatech.com ScryMUD: http://scry.wanfear.com http://scry.wanfear.com/~greear
Quoted 6 lines · Ben Greear
I found this patch:
preempt-kernel-rml-2.4.19-pre2-ac2-1.patchIt applied cleanly...looks like maybe this isn't
the low-latency patch though now that I look at
it a little closer.
Right, it is not. It is the preemptive kernel patch. More information can be found at http://tech9.net/rml/linux
Robert Love
Quoted 8 lines · Ben Greear
I found this patch:
preempt-kernel-rml-2.4.19-pre2-ac2-1.patchIt applied cleanly...looks like maybe this isn't
the low-latency patch though now that I look at
it a little closer.
You are correct sir - that is not the low latency patch, although if you add the lock break patch to the preempt patch, the results are said to be similar to the low latency patch...
Joe
Quoted 5 lines · Alan Cox
Quoted 2 lines
It might be very difficult to fix up the
low latency patch for the latest -ac,You should be able to just dump out the vm part of it - Rik put that into rmap anyuway afaik
Any chance we will see mini-ll or the full-ll patches in an -ac release anytime?
-Erik
--
Erik B. Andersen http://codepoet-consulting.com/
--This message was written using 73% post-consumer electrons--
Quoted 6 lines · J Sloan
The full-on low latency patch from Andrew Morton?
You might want to make some diffs available
since AFIK that would have involved quite a bit
of hand editing to fix rejects...
There is also patch against 2.4.18 with following:
- Latest Andre's IDE driver
- Latest Lionel's SiS IDE driver
- Latest Rik's rmap
- Latest Ingo's O(1)
- Andrew's full and mini lowlatency
- My DRM lowlatency fixes (r128, radeon, mga) (in -ll)
Available at http://www.pp.song.fi/~visitor/linux/
- Jussi Laako
Signature · 2 lines
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