Re: Realtime-preempt performs worse for many threads?

["Jeff V. Merkey" <jmerkey@utah-nac.org>]

Oink Oink. Verified.

Jeff

Carlos Antunes wrote:

>On 11/1/05, Carlos Antunes <cmantunes@gmail.com> wrote:
>  
>
>>On 11/1/05, Esben Nielsen <simlo@phys.au.dk> 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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