[linux-dvb] Re: Thoughts on minimum dvr buffer size calculation?

[Johannes Stezenbach <js@linuxtv.org>]

C.Y.M wrote:
> Jan Panteltje wrote:
> >For those that use a budget card or other high speed
> >card (HDTV) and have mysterious aborts,
> >consider what I just wrote.
> >I will leave it at this. as for me the problem is
> >fixed with the buffer size modification.
> 
> It sounds to me like the size of the buffer is directly related to which 
> kind of card you use.  For those only using a single FF card, it seems to 
> me like the required minimum amount would be less then say for a user with 
> a budget card that can capture an entire transponder.  Didn't I read that 
> FF cards have a much lower bandwidth?

It isn't and this discussion is pointless.

Writes to the filesystem are asynchronous and go through the buffer
cache (unless you use O_DIRECT, O_SYNC or some equivalent).
write() only blocks if you have no more memory for the buffer cache
left (e.g. if you disk is too slow).

The buffer overflow problem is a scheduling or VM problem (writing
to the disk keeps your system so busy that there is no time for your
application to read the data from the dvr device in time). One
could try to write a test program which writes junk data at a
constant rate (say 5MB/s) to the disk and see if that encounters
the same problems (if yes: complain to the VM/filesystem folks).
One could also try if e.g. the realtime-preempt patches fix the problem.

IMHO increasing the dvr buffer size is just a workaround, not
a solution.

Johannes