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Re: [vox-tech] real time in linux
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Re: [vox-tech] real time in linux



On Sat, Sep 29, 2001 at 11:38:59AM -0700, Peter Jay Salzman wrote:
> i came across an article 'low latency in the linux kernel' written for
> o'reilly's website.  here's a quote i find curious:
> 
>   under certain circumstance, it may be possible to run an application
>   in a condition known as setuid root.  as the name implies, this
>   condition sets the user's ID to root status, raising the application's
>   execution to real-time scheduling.
> 
> this is NOT my understanding of setuid root executables.  if this were the
> case, nice-ing a setuid executable would be meaningless since the application
> would get the attention of the kernel whenever it wants attention.

There is a POSIX realtime API that is usable by processes running as root but
it is not realtime in the rigourous, CS sense, which is called hard realtime
to distimguish it from low latency. Lots of people don't get the distinction
between realtime and low-latency (or choose not to in the case of some
marketing depts). Joel explained this better than I can. 

POSIX realtime scheduling allows an app to run at a higher priority than
kernel threads. Nothing more. CDrecord uses this; maybe LAME too. A
preemptable kernel is needed to make this really useful; Linux 2.4 is heading
that direction but is not quite there yet. And no, you don't get POSIX
``realtime'' scheduling merely by setting the suit bit.

-- 
Henry House
OpenPGP key available from http://romana.hajhouse.org/hajhouse.asc

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