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Subject2.6.13 brings buffer underruns when recording DVDs in 16x (was Re: "Read my lips: no more merges" - aka Linux 2.6.14-rc1)
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On Tue, 2005-09-13 at 12:11 -0700, Linus Torvalds wrote:
>
> On Tue, 13 Sep 2005, Mathieu Fluhr wrote:
> >
> > Okay, here is the point: I will have these bloody buffer underruns
> > unless I select a 'Timer frequency' of 1000 Hz in 'Processor type and
> > features' section of the kernel configuration. That's quite
> > understandable, as recording a DVD at 16x requires a throughput of 22160
> > KB/s, which is quite fast.
> >
> > I will have a deep look in the patch, and maybe write a patched patch
> > (Ooooo my god what am I writing ?) in the next few days.
>
> It may just be an application bug too. Too small a buffer, and depending
> on 2.6.x with a 1kHz timer having timers that run faster...
>

According to the MMC documentation, you can thoeriticaly send at most
65535 (16 bits int) blocks in one WRITE(10) CDB. This would means
sending a buffer of ~127 MB on case of writing a mode 1 data track (2048
bytes per block)...

Now, practically, it is really not safe to send more than 64 KB per CDB
(Mostly device related). And with such values, you have the following:
- at 100 Hz -> 64 KB * 100 = 6400 KB/s <=> ~4.62x DVD
- at 250 Hz -> 64 KB * 250 = 16000 KB/s <=> ~11.55x DVD
- at 1000 Hz -> 64 KB * 1000 = 64000 KB/s <=> ~46.20x DVD

I would suggest to leave to default value of the timer frequency to 1000
Hz and to add some more comment in the Kconfig.hz file. (Patch attached)

Mathieu

> Linus
> -
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--- linux-2.6.13.1.clean/kernel/Kconfig.hz 2005-09-10 04:42:58.000000000 +0200
+++ linux-2.6.13.1/kernel/Kconfig.hz 2005-09-13 20:32:35.000000000 +0200
@@ -4,7 +4,7 @@

choice
prompt "Timer frequency"
- default HZ_250
+ default HZ_1000
help
Allows the configuration of the timer frequency. It is customary
to have the timer interrupt run at 1000 HZ but 100 HZ may be more
@@ -35,6 +35,8 @@
help
1000 HZ is the preferred choice for desktop systems and other
systems requiring fast interactive responses to events.
+ 1000 HZ is also required if you want to use large throughput on your
+ burning devices, like when burning a DVD at 16x.

endchoice
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