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SubjectRe: [alsa-devel] Re: [rtl] Low-latency patches working GREAT (<2.9ms audio latency), see testresults ,but ISDN troubles
[yodaiken@chelm.cs.nmt.edu]
> On Sun, Aug 29, 1999 at 10:40:02AM -0700, Brian Swetland wrote:
> > [yodaiken@chelm.cs.nmt.edu]
> > Well, as you point out, BeOS is not a hard realtime system, however
> > the fact that the filesystem, block cache, drivers, etc are all
> > preemptable means you can get some pretty decent response. Interrupt
> > response from timers or IO interrupts in the 30-60us range is certainly
> > doable. It'd be neat to actually put together some benchmarks to
> > compare this sort of thing.
>
> It would be quite interesting. My prediction is that the fully premptable
> nature of the system makes high bandwidth i/o unlikely. I'd really like
> some data though.

Well, if you have a bunch of realtime / high priority threads running,
they quite possibly can cut into your IO performance. If you're doing
large reads/writes that bypass the cache (>64KB or on request via ioctl)
the driver (provided the device is fairly modern) should be able to do dma
into/outof userspace which can be a nice win.

I'm actually working on rewriting an old interrupt latency benchmark
(predating the programmable timers in R4.x) to see if I can get some
real numbers here. Technically interrupts should be fieldable in the 5-10us
range provided there's not a misbehaving driver disabling IRQs for extended
periods of time.

> > Networking on BeOS is not stellar, but it's servicable and performance
> > issues are being worked on. Raw disk IO is actually pretty good, to my
> > knowledge, though obviously real numbers are what you want here and
>
> Raw disk and FS performance are really different. DOS gets decent raw
> disk performance, but our experience is that app ports to RTLinux get
> an enormous benefit from ext2.

I believe Dominic found that BFS tends to see about 5% worse performance
for raw streaming IO than reading/writing the raw device. File creation/
deletion/etc suffers some due to journaling and indices (though indices can
be turned off). Not much seems to beat ext2 for pure speed.

Brian


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