mplayer + heavy io: why ionice doesn't help?

[Denis Vlasenko <vda.linux@googlemail.com>]

Hi,

I noticed that mplayer's video playback starts to skip
if I do some serious copying or grepping on the disk
with movie being played from.

nice helps, but does not eliminate the problem.
I guessed that this is a problem with mplayer
failing to read next portion of input data in time,
so I used Jens's ionice.c from
Documentation/block/ioprio.txt

I am using it this:

ionice -c1 -n0 -p<mplayer pid>

but so far I don't see any effect from using it.
mplayer still skips.

Does anybody have an experience in this?
--
vda