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SubjectRe: a joint letter on low latency and Linux
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>> I can say only that I tried my best to generate higher than 2msec latency
>> spikes on a my old P133, but I failed.
>
>> I have no mathematical proof that the lowlatency patch provides "hard realti
>me",
>> but but empirical testing under extreme conditions show us that works well f
>or
>> doing realtime multimedia.
>
>It's benchmark. Benchmark is great way to spot the problem but it's not very
>usefull to prove that you have solution.

Its a benchmark that started life as a real program, and had to be
gradually reduced to "benchmark" status because several Linux kernel
developers don't have soundcards in their systems.

If linux kernel folk were willing to install the hardware necessary to
run the "real" programs, I could point them to dozens of them in an
instant, all of which would fail when configured for low latency. But
they are not (which is just fine), and so we have to use "benchmarks"
to demonstrate just how messed up things are.

>> The load I created is certainly bigger than the average load of a desktop
>> system, thus even if the lowlatency patch can not called "hard realtime",
>> it is SUITABLE for realtime multimedia.
>
>If it's good for you then use it ! What's the problem here ? If you want it
>to be included in kernel... It's COMPLETELY other story.

It certainly is. A much more IMPORTANT story. As has been pointed out
by many people, Linux right now is completely unusable for any "live"
audio or video work, and has latencies more akin to batch processing
OS's of the 1960's, scaled a little for h/w improvements. We would
like to persuade other people to use Linux, because we think its a
fabulous OS in so many ways, but to do that, it has to be able to do
better than the current ridiculous latency characteristics. To fulfill
that goal, its the kernel that has to be fixed, not just telling
people that a patch exists.

>> the risk that we could loose 1msec of audio during the next 50 days because
>> lowlatency is not hardrealtime is not worth to switch to RTLinux.
>
>> ... just like saying better not take the plane, it might crash
>> such is life
>> :-)
>
>> (we live in a statistical world)
>
>You - may be. Linus - not :-) That's why Linus works (unlike some other OSes).

The problem is: Linux DOESN'T work unless its OK for your
not-too-far-from-1GHz processor to sit around and twiddle its thumbs
for 10s of msecs while chunks of code that made it through the filters
of this list screw around with lengthy codepaths, spinlocks and IRQs.

--p


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