[MPlayer-dev-eng] XV page flipping synchronization

Piotr Neuman sikkh at wp.pl
Sun Mar 14 16:55:12 CET 2004

Hi, I am a newbie in mplayer delopement, so please be patient with me ;).

On my desktop machine (Athlon XP 2000+, Gigabyte VIA VT266a, Radeon 9000, 
XFree 4.3, kernel 2.6.4-ck1) there were some problems in synchronization of 
video output (XV driver) - I could see for example 'tearing' (when double 
buffering was used).

After some investigaton into it with help of Con Kolivas we were able to 
conclude that on my kernel/hw configuration, there is priviledge inversion 
issue (mplayer niced to +1 would work just fine). Such problems are usually 
caused by how programs are written.

It boils down to the usage of XFlush() in libvo/vo_xv.c . The problem is that 
when synchronizing mplayer video buffer with X display, this function can't 
ensure that X server will be given timeslice soon enough.

Mplayer can continue rendering of next frame/frames (when page flipping is 
used) after calling XFlush but X server may not be able to update screen on 
time, so few pages may be 'lost' (flipped at wrong time intervals beetween 
each other).

My patch changes XFlush() call into XSync() which ensures that page will be 
flipped properly on time (ie. when using -double switch), becose this call 
waits till X has reported completion and gives free hand for system scheduler 
to assing timeslice for X (thanks to blocking mplayer - preempting it).

Performance degradation is neglible, according to my limited testing. For 
example for Matrix mirage trailer (matrix_tr_mirage_480.mov) (sound output 
thru alsa1x):

before patching:
BENCHMARKs: VC:  13,746s VO:   1,344s A:   0,954s Sys:  43,573s =   59,617s
BENCHMARK%: VC: 23,0570% VO:  2,2551% A:  1,5997% Sys: 73,0882% = 100,0000%

BENCHMARKs: VC:  11,691s VO:   4,802s A:   0,912s Sys:  42,216s =   59,621s
BENCHMARK%: VC: 19,6089% VO:  8,0539% A:  1,5296% Sys: 70,8077% = 100,0000%

With this change the output is synchronized very well, even though mplayer 
uses usleep() timing (note that 2.6 kernels have clock set to 1000Hz, so 
using rtc is uneceseary - A-V sync is ~ +- 0,005 at worst). The issue of 
prority inversion is gone.

The patch also contains removal of unneceseary calls to XFlush function just 
before XSync is called (XSync() alone suffices).


Piotr Neuman
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