Re: [PATCH] [request for inclusion] Realtime LSM

[Ingo Molnar <mingo@elte.hu>]

* Jack O'Quin <joq@io.com> wrote:

> The numbers I reported yesterday were so bad I couldn't figure out why
> anyone even thought it was worth trying.  Now I realize why. 
> 
> When Ingo said to try "nice -20", I took him literally, forgetting
> that the stupid command to achieve a nice value of -20 is `nice --20'.
> So I was actually testing with a nice value of 19.  Bah!  No wonder it
> sucked.
> 
> Running `nice --20' is still significantly worse than SCHED_FIFO, but
> not the unmitigated disaster shown in the middle column.  But, this
> improved performance is still not adequate for audio work.  The worst
> delay was absurdly long (~1/2 sec).
> 
> Here are the corrected results...
> 
>                                  With -R        Without -R      Without -R
>                                (SCHED_FIFO)     (nice -20)      (nice --20)
> 
> ************* SUMMARY RESULT ****************
> Total seconds ran . . . . . . :   300
> Number of clients . . . . . . :    20
> Ports per client  . . . . . . :     4
> Frames per buffer . . . . . . :    64
> *********************************************
> Timeout Count . . . . . . . . :(    1)          (    1)          (    1)
> XRUN Count  . . . . . . . . . :     2             2837               43
> Delay Count (>spare time) . . :     0                0                0
> Delay Count (>1000 usecs) . . :     0                0                0
> Delay Maximum . . . . . . . . :  3130 usecs    5038044 usecs   501374 usecs
> Cycle Maximum . . . . . . . . :   960 usecs      18802 usecs     1036 usecs
> Average DSP Load. . . . . . . :    34.3 %           44.1 %         34.3 %    

what kind of non-audio workload was there during this test? 43 xruns
arent nice but arent that bad either.

plus, is it 100% sure that all audio threads inherited the nice --20
priority - including the client threads? Nornally jackd does a
setscheduler for the client threads so that they get boosted to
SCHED_FIFO, but there is no parallel to that in the nice --20 case, did
you do that manually (or did you start the clients up from the nice --20
shell too?))

If the nice --20 priority setup is perfect and there are still xruns
then could you try the following hack, change this line in
kernel/sched.c:

 #define STARVATION_LIMIT        (MAX_SLEEP_AVG)

to:

 #define STARVATION_LIMIT        0

this will turn off starvation checking, for testing purposes. (to see
whether there's anything else but anti-starvation causing xruns.)

	Ingo