Re: Realtime-preempt performs worse for many threads?

[Carlos Antunes]

On 11/1/05, Carlos Antunes <cmantunes@xxxxxxxxx> wrote:
> On 11/1/05, Esben Nielsen <simlo@xxxxxxxxxx> wrote:
> > On Tue, 1 Nov 2005, Carlos Antunes wrote:
> >
> > > Hi!
> > >
> > > I've been developing some code for the OpenPBX project
> > > (http://www.openpbx.org) and wrote a program to test how the system,
> > > responds when hundreds of threads are spawned. These threads run at
> > > high priority (SCHED_FIFO) and use clock_nanocleep with absolute
> > > timeouts on a 20ms loop cycle.
> > >
> > > With the stock 2.6.14 kernel, I get latencies in the order of several
> > > milliseconds (but less than 20ms) when running 1250 threads
> > > simultaneously. However, when I switch to a kernel patched with
> > > realtime-preempt latency increases to several hundred milliseconds in
> > > many cases.
> >
> > There is only one explanation:
> > Some of the operations (task switch, nanosleep etc.) are more expensive in
> > the RT kernel. Thus your 1250 threads spend 100% CPU doing what they do.
> > You therefore get very bad latencies.
> >
>
> Esben,
>
> Thanks for replying. Let me chalenge this assumption of yours, though.
>
> I just ran a test with those 1250 threads (all they do is sleep for
> 20ms, wake up, increment a number, and repeat the process). The CPU
> was 86% *IDLE* while running this. One thread took 1.3 seconds to wake
> up once. Do you think this is, well, normal, given how RT is supposed
> to operate?
>

Esben,

If, instead of SCHD_FIFO, I use SCHED_OTHER, I get max latency in the
order 13ms running those 1250 threads. With SCHED_FIFO (the only
change), I get 1.3 seconds. Makes sense to you?

Thanks!

Carlos

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