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SubjectRe: a joint letter on low latency and Linux
Hi!

> > On Wed, 5 Jul 2000, Richard Gooch wrote:
> > > RTLinux doesn't have to be conservative this way. It doesn't have to
> > > work around broken hardware ("Oh, it hangs? Go use decent hardware").
> > > This is a key point I'm finding difficult to get across.
> >
> > Youre saying we cant do better than 4000ms? Ouch.
>
> I'm saying we can't do better than N ms, where N is an unknown
> quantity (because all drivers and core kernel code has not been
> audited, and said auditing would take man-years and be an ongoing
> project). And people have reported large values of N. Apparently the
> PS/2 driver has N >= 50. There may be other stuff that is much worse.
> I think Andrew Morton measured something with N = 160.

With fbcon on machine with loaded PCI bus (usb modem), N can be ~500,
even more with really full screen. You know, fbcon is running with
interrupts disabled.

> But people should not fool themselves into thinking that anybody has a
> clue how big N could be. Even with a specific hardware combination,
> there may always be some kernel slow algorithm that is lurking, ready
> to blow out N.

Pavel
--
I'm pavel@ucw.cz. "In my country we have almost anarchy and I don't care."
Panos Katsaloulis describing me w.r.t. patents at discuss@linmodems.org

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