Subject: Re: [linux-audio-dev] Re: linux-audio-dev Digest, Vol 2, Issue 24
From: Roger Larsson (roger.larsson_AT_norran.net)
Date: Tue Nov 18 2003 - 09:22:45 EET
On Tuesday 18 November 2003 02.41, Jack O'Quin wrote:
> Fernando Pablo Lopez-Lezcano <nando_AT_ccrma.stanford.edu> writes:
> > > - Can not provide mlockall since it has no pid parameter - the
> > > monitor can't do it for another process. (I would say that it is
> > > unlikely that pages used in a tight audio loop would be thrown out
> > > - big buffers might... Add additional page reads?)
> >
> > Well, I'd say this is the showstopper. We really need this. "Unlikely"
> > is not enough. Eventually memory will run out and the wrong page will
> > fault and we get a click. We have to be able to lock memory......
>
> Agreed. IMHO, mlock() is mandatory, certainly for JACK applications.
Problem is - why doesn't most distributions even ship with wrappers suid
to be able to start the application with SCHED_FIFO/RT/mlock?
- It is due to risks of local Denial Of Service attacks (intentional or
unintentional)
So with any scheme that opens up these holes you have to deliver a way
to protect from the downsides.
My monitor protects from CPU overuse, but what about memory?
How to protect from an application that mlockall(MCL_FUTURE) and
has a memory leak?
One important thing to remember - if you like to get broad acceptance
you have to suggest a solution that solves these problems. I would say
that the rt_monitor or some other means to do the same thing is
mandatory to get that kind of acceptance.
>
> The big difference between realtime and most other kinds of
> performance work is that it focuses on tuning the "worst case", not
> the average. Paging works fine on average, but in the worst case your
> recording session gets blown.
SCHED_FIFO does not make ANY guarantees on "worst case"!
>
> Otherwise, a good solution. Perhaps adequate for some applications.
But at the same time SCHED_FIFO is adequate for most applications.
See my point? :-)
/RogerL
-- Roger Larsson Skellefteċ Sweden
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