Re: Realtime-preempt performs worse for many threads?

[Jeff V. Merkey]

Oink Oink. Verified.

Jeff

Carlos Antunes wrote:

> 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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