loop device and horrible latency

[Zenaan Harkness]

(Please CC me on replies: not subscribed to list.)

I think that when using the loopback device there
are significant problems with achieving low latency.
I'm wondering whether this is known or if there is something else
causing my mp3 dropouts. My experience follows:

I've been setting up a local debian distribution mirror on my laptop:
DELL Inspiron 8100, 384MB, 60G drive, Radeon Mobility 7500 graphics,
PIII 1GHz, Maestro 3i sound.
Kernel 2.4.18.

To set up my Debian mirror, I'm using apt-move, using apt-move movefile
on each .deb on each of my cd's to get them into my mirror.

The 3rd CD in the set (February debian 'testing' - 8 cds) was thrashing
and taking much longer than the first two - the CD drive sounded like
it was moving the head from inside to outside and back for each file or
something... anyway:

I decided to dd the cds and loopback mount the images. Which I did
(ended up doing this for each of the 8 cds). loop.o is a module. Playing
an mp3 while running apt-move movefile resulted in horrible dropouts,
so:

First I tried the preempt patch combined with lockbreak patch,
configure, rebuild, reboot and then ran my 'test' again, then tried the
test with the 'nice' command as follows:

mount /imgN /mnt -o loop,ro
nice find /mnt -type f -name "*.deb"|nice xargs nice apt-move movefile

in parallel with:

nice -n -19 mpg123 some.mp3

Consistently ugly gaps in sound, so I figured the preempt patch may have
some problems at this stage (hand't read anything on it at that point,
other than finding its existence and location to download), so:

Second I tried the a.k.morton lowlatency patch (slightly different
version to kernel available as of yesterday, but patch figured it out
with a few line number shifts).

Ran the above test and had very similar experience. Realised I'd
forgotten to turn on the option in make menuconfig for low latency, so
did so, recompiled again and rebooted ... same problems.

BTW, apt-move does a bunch of file related stuff - looking at the files,
then copying the files (from my loopback mounted cd image, to a
partition on my drive).

I had the same results with an image on the same and on different
partition as the destination partition to which the files were being
copied.

So I got a little suspicious at this stage and decided to try a test
that I read (I went reading a bit) -> while playing mp3, do:

make clean && make bzImage

MP3 played flawlessly. At this point I was running with akm lowlat patch.
I can also run an mp3 and swap workspaces, windows, VTs, etc - holding
down the respective 'change' key so that it changes as fast as possible:
never a skip in the slightest. During these tests I'm not doing the
loopback thing though. So I'm wondering if anyone else has had such
experience.

Again, please CC me with replies.

thanks
zen