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On Wed, Dec 02, 2009 at 03:41:05PM +0100, fons@kokkinizita.net wrote:

quoted text
> On Wed, Dec 02, 2009 at 08:19:06AM -0500, Paul Davis wrote:

> > On Wed, Dec 2, 2009 at 7:37 AM,   wrote:

> >

> > > This change ensures that threads using the same period

> > > will have the same priority, even if they are not in

> > > the same client. This is absolutely necessary in order

> > > to make two or more such clients work together smoothly

> > > and have 'fair' scheduling. This should become some sort

> > > of 'standard' if more apps start using such schemes.

> >

> > my understanding of SCHED_FIFO says that it doesn't really work this

> > way. if you want to use equal priorities to ensure fair scheduling

> > among RT threads, you want SCHED_RR so that the kernel scheduler can

> > intervene and ensure roughly equivalent scheduling. if you use

> > SCHED_FIFO then every thread of the same priority will run until it

> > voluntarily yields. is this what you mean by "fair" scheduling? it

> > seems to me that whether this works or not depends entirely on what

> > the threads are actualy doing (i.e. how long before they are

> > guaranteed to yield the processor).

>

> In the general case this would be correct, SCHED_FIFO

> does not in any way ensure that threads get a 'fair

> share of time'. But this is not the general case.

>

> In cases such as the one discussed, the threads are

> triggered at a divisor of Jack's period rate. Each

> time they get a quantum of work, perform it, and

> wait for the next trigger. They do not try to run

> for as long as they can.

>

> The threads do not compete on CPU time: the assumption

> is always that there is enough CPU power to perform

> all the work. If this is not the case the whole thing

> fails anyway.

>

> The 'competition' is on the order in which they are

> allowed to run, and for that priority is the only

> criterion.

>

> The objective of the priority scheme is to allow all

> threads to meet their deadline, not to give them equal

> CPU shares. They don't need equal CPU shares anyway,

> in most cases the bulk of the work is done by the

> lowest priority ones.

>

> An example should make this clear. Assuming the

> period size is 256, running the 'Great Hall' reverb

> in jconvolver, configured for a minimum partition

> size of 256 (no processing delay) will result in the

> following threads:

>

> (you can use -v to see this)

>

> prio =    0, offs =      0,  parsize =   256,  npar =   3

> prio =   -1, offs =    768,  parsize =   512,  npar =   6

> prio =   -3, offs =   3840,  parsize =  2048,  npar =   6

> prio =   -5, offs =  16128,  parsize =  8192,  npar =  13

>

> The first one is Jack's process thread, this will do the

> work for 3 partitions of size 256. The second one will do

> 6 partitions of size 512, it will be triggered every two

> periods. The next one runs every 8 Jack periods, the last

> one every 32.

>

> The last three are required to have completed their work

> when the next trigger arrives. Their output is actually

> only required later, 3 periods after the start for the

> size 512, 15 periods for the size 2048, and 31 for the

> last. This gives the whole system some robustness it

> would not have if everything was planned 'just in time'.

> Note that the extra margin is small for small partition

> sizes and quite big for the larger ones.

>

> Now if the same reverb is configured to use a minumum

> period size of 4096 (because the user can tolerate the

> delay and wants less CPU load) this would look different:

>

> prio =   -4, offs =      0,  parsize =  4096,  npar =   2

> prio =   -5, offs =   8192,  parsize =  8192,  npar =  14

>

> As you can see the priorities match the partition size.

> In this case no work is done in Jack's thread.

>

> Now assume that I would use the scheme of version 1.0.0.

> Then the priorities would be

>

> -1   512

> -2  2048

> -3  8192

>

> and

>

> -1  4096

> -2  8192

>

> respectively. This would mean that the size 4096 thread

> of the second instance would have the same priority as

> the size 512 thread of the first. The time required for

> the size 4096 calculations could easily be longer than

> the period of of the size 256 thread. This would result

> in the size 256 thread being late every time it has to

> wait for the other to complete, which could happen every

> 16 periods.

>

> Matching priorities to period size removes this problem,

> and ensure that the most urgent work will always be done

> first.

>  

Does the "partition" parameter in the conf file affect this?
On the reverbs I've been using successfully with freewheeling mode, the "partition" is set to 2048, which is very high compared to the setting in the sample reverbs tthat come with jconv.
-ken

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