Mplayerosx

I have problems with stuttering and video/audio desync when playing videos over wireless network (macbook that is 1m away from airport express), when I don't have the same problems on the wired network. By playing video, I mean that I have an mkv or avi file exported via CIFS from a NAS running samba.

I've done what I know of in terms of tuning the macbook (eg. setting net.inet.tcp.delayed_ack), but I'd be pleased to try any suggestions.

I've also done what I know of in terms of tuning mplayer (ie. setting -cache 16384)

I've observed during playback that I'm reading at ~ 660KB/s, and I believe that the limited throughput is linked to the problem.

Yet, I'm pretty sure that the wireless network has more than sufficient capacity.
ie. cat /Volumes/share/path/to/file.mkv > /dev/null yields in excess of 2 MB/s network throughput.

So, I tried to investigate what bottlenecks might exist in how mplayer reads the file, as opposed to how cat would read the file. Using sudo dtruss -n mplayer, I get a constant stream of something like the following:

24741/0x628e3: read(0x3, "<d5)\272\246]\322,K\215\355\266\003^\377<}yo.\206\246\244\t\236\371'\303\0026\351i\303\027\270n#\345a\266\311\256\003f\302\207$\233\203\270qE\374\244\333\206#R\"\214\tp\307\273_,\230\\\322\204+\326\020\177\326\177\222\241Z\250\371.\364\257\200\264\266L\275\177\024T\250\354q8\372\361W\253\320\255k;\333\217#\270\312\240\265\032\375\267\251\004\214{\340\v7@\365\234M\236\035zz\016%^~\310\022\263\3044\304\342\t\265y5R\227qD\233\316\032vX\247\271.:\"\322\251\232\035v\311\340\242A\003\376\020\273\204\360\235\324\274+\373C;\313\374\001\0044\256\262\260\204/\212:i\305\340\254\217g\336\343<\210\017\240v\210:\260\323\004w\252\372\370\26602\331Y\017)bO\032\221\273{m\274\350v\215\221\321\221@\2152\366\0", 0x800) = 2048 0
24741/0x628e3: read(0x3, "\023\344\313!\3465#\003\301\315:3\374D4\235\247\230npf\t\304\261ef(\3044\fD\326\363\327($\\\262-\374-\216*e\332b8\n\025\244S4\313\243\342\026\246\266\363\306\306+\310\021\233\262\317\026\343\tD^r:\223\341u\257 \035\371\245\0041\246\322%Z\3504n\344#\"uAT\352q\374\a;w#\212L\224\373\217,\326\201\242\2017\006\21688\024B9v\350\177&\346\345\205$2}3iA\213\025\335\361\373\0220W\214Y\001\306Z\220\272\261\227\001\030\366\215w\244\333\001\fsLQ&\357F\025\r\0347bP%\226a5\002\265\0", 0x800) = 2048 0

From the read(2) manpage at http://www.manpagez.com/man/2/read/, the 0x800 indicates that a buffer size of 2048 bytes was allocated for the read, and the return value of 2048 indicates how many bytes were successfully read. Note that this 2048byte read buffer is not linked to the 16384 KB cache that I had specified.

Could this be the cause of the problem? Looking at the dtruss output for the cat command, the buffer size is 0x20000, or 131072 bytes, which is significantly larger than 2048.

I did some other tests that suggest that this read buffer size really could be the problem.

dd if=/Volumes/share/path/to/file.mkv of=/dev/null bs=2048 yields consistent network speed of ~ 660KB/s, just like mplayer.

I hypothesize that the read syscall blocks, and so the maximum theoretical throughput would be bounded by buffersize/network latency. On my wireless network, I get pings to the NAS of approximately 2.5ms. ie, 0.0025ms

Playing with other values shows that the read buffer size has a visible relationship to network speed, up to a certain point.
bs (B) approxthroughput (KB/s) theoretical bs/latency (KB/s)
512 150 204
1024 400 410
2048 650 819

Is it possible to increase the read buffer to something that would work in high bandwidth, high latency environments? If possible, I'd suggest going with the value used by cat (128KB or 131072 bytes), so that it can 1.3MB/s is still achievable in a long distance network with 100ms latency.