[linux-dvb] Re: [PATCH] Skystar2 pid filters reorganization

[Roberto Ragusa <r.ragusa@libero.it>]

On Sat, 29 Nov 2003 15:59:21 +0100
Holger Waechtler <holger@convergence.de> wrote:

> Roberto Ragusa wrote:

> > In relation to "one of the first checks", do you think that the swfilter
> > code is fast enough to let us just open_whole_bandwith when we're out of
> > hw filters?
> 
> sure, why not? All other decoder-less PCI cards wotk reliably the same 
> way...

Maybe I didn't explain well what I meant.
I'm not saying "will the hardware be fast enough?".
I'm saying, instead, the following:

skystar2 filter emulation:
- let S2EMU be the time for every packet examined (and then accepted or refused)

swfilter:
- let SWF be the time for every packet examined (and then accepted or refused)

With skystar2 filter emulation, processing one packet takes:
  S2EMU + SWF * fraction_accepted

With swfilter only, processing one packet takes:
  SWF

"Swfilter only" is better if SWF < S2EMU / ( 1 - fraction_accepted )

Now, what we have is
  S2EMU = a small time (comparison between pid value and an array of 10 or
            20 elements)
  fraction_accepted = 0.1 or 0.2

so the question is if SWF is really less than 0.9 S2EMU.
This is where I want your opinion.

You said that it is one of the first tests, but I'm wondering if it is
really fast enough to be the best choice.
For example, if the packet is copied to a buffer, SWF is certainly
very big. Even if there is no copy, I guess that S2EMU is too small
to be beaten.

> I'd suggest to implement and activate the PID filters mostly to save PCI 
> bus bandwidth, keeping the PCI latency low with many cards in your PC is 
> usually a much more serious problem than CPU load caused by the software 
> demux.
> 
> And in most environments you don't need the entire 38MBit Stream but 
> audio, video and some section data streams which sum up to 4-10MBit...

This is perfectly right.
If we can filter before PCI traffic, we do it.
But when we have no choice (too many pids wanted) the problem I described
above takes place.


-- 
   Roberto Ragusa    r.ragusa at libero.it


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