Rick Steeves wrote:
>Roger Heflin wrote:
>> That would be network then.
>>
>> THe mvpmc does have issues if the block size is too large, it loses
>> packets because the network chip is rather cheap.
>
> I don't see how it's network. I'm moving effectively the same amount of
> data across the network using either method (myth protocol or CIFS
> directly), right? That it works with CIFS means that the network is
> handling the load just fine, right?
CIFS, at the application layer, may bundle data into chunks that are
smaller, or otherwise different from the MythTV protocol, and apparently
better handled by your network.
That's, of course, just speculation, but Roger points out how with NFS
you can adjust the block size, and it makes a difference.
> Do you mean blocksize of the file system?
No, he is referring to what gets bundled into a network packet.
> I don't see how using CIFS successfully doesn't remove the question
> of the server's performance, and bring the entire issue back to the
> mvpmc code that handles receipt of data from mythtv protocol.
While the code may have problems, it does seem to work for the vast
majority of users most of the time. So there must be something unique to
your environment that is different.
Have you looked to see if the MTU is set the same on the MVP as on your
server?
# ifconfig
eth0 Link encap:Ethernet HWaddr 00:...:28
inet addr:192.168.0.242 Bcast:192.168.0.255 Mask:255.255.255.0
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:1899738 errors:3924 dropped:0 overruns:3924 frame:0
TX packets:840712 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:2710291352 (2.5 GiB) TX bytes:0 (0.0 B)
Interrupt:27 Base address:0xd300 DMA chan:1
This also could be related to TCP window scaling, where CIFS may have
enough acknowledgment points in its protocol to naturally throttle the
connection, whereas a simpler protocol dependent on TCP windows would
have problems if something is breaking the TCP window scaling, resulting
in overruns.
See:
http://lwn.net/Articles/92727/
Ways to work around the problem:
http://wiki.archlinux.org/index.php/Configuring_Network#The_TCP_window_scaling_issue
(you can temporarily disable window scaling on your server)
Background on TCP flow control:
http://www.linuxplanet.com/linuxplanet/tutorials/6539/1/
Linux TCP tuning (you should be able to experiment with some of these
settings on the MVP side):
http://fasterdata.es.net/TCP-tuning/linux.html
-Tom
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