[mythtv-users] MythTV problems with 3.7.x and 3.8.x kernel

[Richard]

On 26/02/13 03:22, Neil Salstrom wrote:
> On Sun, Feb 24, 2013 at 8:48 AM, Larry Finger <Larry.Finger at lwfinger.net> wrote:
>>
>> I work in driver development for wireless devices and need to work with the
>> latest cutting-edge kernels. As such, I have bisected for regressions many
>> times. Note: The mainline tree only knows about the .0 version - any
>> follow-on fixes are kept in a separate repository. Mainline refers to
>> versions as v3.7, not v3.7.0.
>>
>> The 'git bisect' tool is very powerful; however, when bisecting between
>> releases such as 3.6 and 3.7, the first set of commits frequently cause the
>> most problems with build errors and run-time faults that have nothing to do
>> with the bug under investigation. I would suggest starting with the git
>> command 'git checkout v3.7-rc1'. If the resulting kernel is OK, then you
>> bisect between v3.7 (bad) and v3.7-rc1 (good) and you will avoid the worst
>> of those problems. If -rc1 has the problem with the DVD, then the bad
>> version is v3.7-rc1 and good is v3.6.
>>
>> Good luck,
>>
>> Larry
>
> SOLVED!!!!!!!!!!!!!
>
> After ~12 or so bisect iterations this issue has been solved.  I don't
> pretend to understand why it works but it does.
>
> Oddly, the change happened in drivers/net/loopback.c starting with the
> 3.7 series kernel (commit 0cf833aefaa85bbfce3ff70485e5534e09254773).
> Here is summary:
>
> net: loopback: set default mtu to 64K
>
> loopback current mtu of 16436 bytes allows no more than 3 MSS TCP
> segments per frame, or 48 Kbytes. Changing mtu to 64K allows TCP
> stack to build large frames and significantly reduces stack overhead.
>
> Performance boost on bulk TCP transferts can be up to 30 %, partly
> because we now have one ACK message for two 64KB segments, and a lower
> probability of hitting /proc/sys/net/ipv4/tcp_reordering default limit.
>
>
> The odd thing is there is no networking involved in playing DVDs on my
> system.  It's a combined frontend / backend and the media drive is
> local to the system.  Going back to the original mtu of 16346 bytes
> fixes the stuttering when a DVD is played.  I've tested this fix with
> both 3.7.9 and 3.8.0 kernels.
>
> Does MythTV mount a dvd or .iso over a loopback device?

Hi Neil,

You can search your frontend logs to see how a DVD (image) was opened. 
Grep for "Opened DVD".  This string is output under 'General', so you 
should see it without having to specify anything on the command line.

If you use storage groups, you should see a "myth://" path (even on a 
combined FE/BE), otherwise it should be a local path.

I don't know, but I assume that storage group access will always be via 
the network, even if it ends up on the same machine.  It may well get 
sent via the loopback device, though.  I'm not clued up enough on 
networking magic to do more than guess here, though.

A local, physical drive should be accessed directly so I don't know why 
playback from there should be effected by any network settings.

Richard.