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From: | Frank Brickle |
Subject: | Re: [Discuss-gnuradio] SCHED_FIFO and NPTL |
Date: | Wed, 08 Mar 2006 09:46:54 -0500 |
User-agent: | Mozilla Thunderbird 1.0.6 (Windows/20050716) |
Eric Blossom wrote:
Bottom line, it hasn't actually been proved that running SCHED_FIFO will squash the existing latency and continuity problems, so I'm not at all sure much is missing without that capability.Frank, is this a statement or a question?
It's a statement. I don't have any real proof at the moment but I suspect SCHED_FIFO is a bit of a red herring. I haven't got proof because the couple of times I tried to actually measure a difference I tripped over additional activity that needed to be accounted for, like, for example, periodic journaling filesystem updates.
The other unfortunate consequence was that some kinds of X action on the desktop, like switching between multiple desktops, would get deferred -- it got real sluggish -- and then all the display activity would happen in a burst. Sometimes that would choke the audio worse than an occasional xrun. Oh, and the interaction with audio subsystem buffer sizes was unpredictable.
I'm suggesting that the current non-RT scheduling is very delicately balanced and there's no single, universal fix. What's more, I conjecture that the worst culprit is disk activity, specifically paging, and that the cost/benefit fix for that is running off ramdisk :-)
It would be *wonderful* to be wrong about this. As I said, it's just a strong hunch.
I'm not sure if the JACK FAQ is up to date or not.
Usually not :-)
Do you have data on the success or failure of folks running JACK on ALSA using NPTL and if they are able to get sufficiently good performance? I guess Ardour users would be a good test case. How about the dttsp stuff under Linux?
Again, it ain't conclusive, but it looks like xruns are not a problem with DttSP under Linux. You're absolutely right, Ardour users are the ones to ask. I finally have it installed here and can check it out first hand, at long last.
Frank
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