[MPlayer-users] DVD playback issue on older notebook with not so old Linux kernel

[Erik Auerswald]

Hi,

I'd like to describe a DVD playback issue I encountered, and how to work
around the problem, just in case someone else runs into it.

Since upgrading an older notebook (with HDD, no SSD) from Ubuntu 16.04 LTS
to Ubuntu 18.04 LTS, I could no longer play a complete DVD with MPlayer
without "hanging" playback.  The picture would freeze for a short,
but noticable time, at random moments.

This issue can be prevented by disabling swap:

    sudo swapoff -a

I would do that for a dedicated media player PC, but I'd rather avoid
this for a general purpose notebook system.  I disable swap just before
playing a DVD now.

The problem lies in the use of swap at all, which impacts the system
heavily, instead of freeing pages from the page cache, which does not
impact the system at all for the streaming workload of DVD playback.

Adjusting the "vm.swappiness" value does not help, swap is still
preferred over freeing page cache pages, even with vm.swappiness=0.
A quick glance at the current Linux kernel code suggests that the meaning
of this value has changed dramatically through the years.  Currently,
it is only checked for equality with zero.

Clearing the page cache before DVD playback does not suffice on the
above mentioned notebook, because it does not have sufficient RAM for
all of the DVD, MPlayer, and a desktop environment.

The issues could probably be prevented by upgrading the hardware:

  1. Having sufficient RAM to hold all of the DVD in the page cache,
     as well as all the software used during DVD playback (MPlayer,
     desktop environment, ...).  16GiB should suffice for DVD playback.
     This is not possible for the old notebook mentioned above.

  2. Replacing the HDD by an SSD might work as well.

A 2016 LWN article, "Reconsidering swapping", describes how back then the
Linux memory subsystem was overhauled to make better use of swap on SSD.
While this was supposed to not significantly impact HDD users, since it
relies on using swap before deciding on how much to use it, where before
swap was not used unless really necessary, this does affect HDD users.

The basic idea can be gleaned from
https://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1160330.html:

    "However, when recycling page cache incurs a higher cost in IO than
     swapping out a few unused anonymous pages would, it makes sense to
     increase swap pressure."

The streaming IO workload of DVD playback adds all of the movie data to
the page cache.  This results in filling the RAM, and now RAM contents
can be written to disk, i.e., swapped out, instead of just dropping
older page cache entries.

>From reading the kernel source, it may be possible to prevent swapping
by placing MPlayer into a memory cgroup with memory.swappiness for the
cgroup set to 0.  I have not yet tried this.

Many years ago, MPlayer developers looked into using something like
madvise(2) to optimize MPlayer's memory usage, but at least back then
it did not look promising.  Thus MPlayer relies on the kernel providing
useful default behaviour.  It would still rely on the kernel providing
useful specific behaviour, which was deemed not the case back when.
This may have changed for the better, but this is far from certain.

Thanks,
Erik
-- 
Software engineering is programming integrated over time.
                        -- Titus Winters