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

[Erik Auerswald]

Hello Alexander,

On 17.05.20 13:07, Alexander Strasser wrote:
> On 2020-05-15 11:05 +0200, Erik Auerswald wrote:
>> 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.
> 
> Did you have a look at the caching settings?
> In particular -cache and -cache-min ?

Yes, I did.  I am using a 64MiB MPlayer cache for DVD playback with the
default -cache-min setting.

I have been using these settings for years on the exact same hardware.
The playback issues described in this thread are relatively "new".
(I reinstalled the notebook with Ubuntu 18.04 about a year ago.)

Using no MPlayer cache for DVD playback, e.g., for dvdnav://, does not
work well on that notebook.

The playback problems usually occur late in the movie, even after
pausing the playback (which should give the cache time to fill).

Swap stays usually empty on that system, but directly after the first
stutter during DVD playback, swap is used.  After each stutter, more
swap is used.  Disabling swap has so far reliably prevent the issue.
The stuttering "feels" just like Linux's whole system stalls on write
back.  Since I do not have an HDD LED on the notebook, I cannot easily
see if HDD activity and DVD playback problems coincide.  I do think that
my conclusions are plausible (and disabling swap works for me ;).

> [...]
>> 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.

I have tried this, but it proved not to be sufficient.

While putting a memory cap on the cgroup (e.g., 512MiB) and setting
memory.swappiness=0 for the cgroup can prevent an MPlayer process placed
there to induce swapping, sometimes the page cache contents are not
attributed to MPlayer.  When playing large video files (not DVDs) from
an NTFS filesystem mounted via FUSE, page cache grew beyond the cgroup
limits.  When playing DVDs, sometimes page cache use of DVD data was
limited by the cgroup memory controller, but once it was not (ejecting
the DVD freed about 5GiB from the page cache, so it was DVD data).

There is still the problem of other activity on the system beside
MPlayer, thus limiting how MPlayer alone affects the page cache cannot
always be sufficient.

>> 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.

I was probably thinking of posix_fadvise(2).

The situation may have changed since then, but I did not check
(https://lwn.net/Articles/480930/).

With MPlayer providing a specialized caching layer (unless -nocache is
used) it may seem sensible to try to minimize page cache impact when
-cache is used.  This could help in general, but would probably not
suffice to reliably prevent the swap issue I have described.

Before Ubuntu 18.04, I was using encrypted home directories instead of
full disk encryption.  Then I relied heavily on the page cache for
decent performance, and playing a DVD totally trashed the page cache,
but DVD playback worked without stuttering.  The full disk encryption
of Ubuntu 18.04 seems much more performant, because this general
performance issue is gone.

But I doubt that the results of MPlayer based page cache control can be
significantly better than using a memory cgroup to constrain MPlayer.
It would be more convenient, though.

Additionally I tried using nocache (https://github.com/Feh/nocache),
but that did not have any observable effect on DVD data in the page
cache.  It did not prevent swapping and the swapping related pauses.

Thanks,
Erik