Re: [Jack-Devel] Real Time extensions for JACK
On Fri, Apr 15, 2011 at 2:49 PM, Bearcat M. Sandor
<[hidden]> wrote:
> On 4/14/2011 7:55 AM, John Rigg wrote:
>>
>> JACK requires realtime scheduling privileges, which the standard Linux
>> kernel has been able to provide for a long time now.
>>
>> The -rt patch for the kernel provides realtime preemption, which is
>> not required by JACK.
>>
>> John
>
> Can someone explain what the difference between realtime scheduling and
> realtime preempting is? I know what the words mean, but what does a system
> do differently in each case?
simple, short answer limited to SCHED_FIFO realtime scheduling:
* a thread that is set to run with SCHED_FIFO will run forever, or
until it blocks
(e.g. it goes to sleep or does file i/o). it will also be
temporarily interrupted
by an interrupt handler, for as long as the handler runs.
this is totally unlike normal scheduling, where threads will be
stopped so that something else can run for a while.
that's realtime scheduling, and access to it is "controlled" on linux
(not on OS X)
realtime preemption is a bad name, but what is generally being
referred to is how interrupts are handled. in a regular kernel, when a
device
interrupts the CPU, the handler will run right away and will continue
to run until its finished. in kernels with threaded interrupt handlers
(the most important part of "realtime preemption"), only a tiny part
of the handler will run, and the rest of it takes place in a thread
whose priority can be controlled. this means that bulk of the work
done by an interrupt handler for, say, a video device or a keyboard or
network interface can be explicitly prioritized to be "less important"
than a SCHED_FIFO thread, meaning that even interrupts from these
devices will have very little impact on the execution of the
SCHED_FIFO thread's code. there are other aspects to "realtime
preemption", but its not really worth getting into them here.
1302893811.25967_0.ltw:2,a <BANLkTi=dUpFM_E1NfsQjz0Ud3jNWK25_dw at mail dot gmail dot com>