[mythtv-users] High mythbackend CPU usage during DVB-T recording (Australia)

Alex Tomlins alex at tomlins.org.uk
Sun Jul 14 13:02:31 UTC 2013


On 14/07/13 11:57, Christian Hack wrote:
>
> Hi everyone
>
> Having some issues with my latest incarnation of MythTV.
>
> It’s mythbuntu based from 12.04 and updated to the latest everything. 
> Mythbackend reports as “mythbackend version: fixes/0.25 
> [v0.25.3-48-g3a1eff8] www.mythtv.org <http://www.mythtv.org>”
>
> CPU is an i3-2100 on an ASRock B75-PRO3-M motherboard, 3GB RAM, 3TB 
> main drive and a couple of other smaller ones. Playback is VDPAU which 
> works OK.
>
> hdparm –t reports speeds of 60-70MByte/sec no problem on all drives 
> and only 4-5MB/sec when recording as expected. There seems to be no 
> actual issue with recordings. There are no corruptions or dropouts 
> ever that I notice.
>
> Tuners are two dual tuner “USB Lifeview TV Walker Twin” devices for a 
> total of 4 DVB-T tuners. I have 4 virtual tuners on each although 
> rarely do they ever get exercised to the full extent. Helpful for 
> overlapping recordings though.
>
> My problem is that when a recording is running I’m seeing quite high 
> CPU usage from mythbackend. A HD show (1440x1080) will take 60-80% CPU 
> per recording. When a couple of recordings are going I see something 
> like the top output below (2 x HD and 1 x SD recordings) with a load 
> average to match. DVB-T recordings really shouldn’t need much/any CPU. 
> Load average occasionally even gets up to 8-10 or more.
>
> top - 20:46:06 up 1 day,  8:55,  3 users,  load average: 5.29, 5.88, 6.49
>
> Tasks: 211 total,   2 running, 208 sleeping,   1 stopped,   0 zombie
>
> Cpu(s): 20.4%us, 29.0%sy,  0.1%ni, 45.4%id, 5.1%wa,  0.0%hi,  0.0%si,  
> 0.0%st
>
> Mem:   3023632k total,  2953156k used,    70476k free,    12280k buffers
>
> Swap:  9765884k total,   447804k used,  9318080k free,  1418564k cached
>
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>
> 4197 mythtv    20   0 2927m 121m 6040 S  202 4.1   1028:10 mythbackend
>
> 8672 mythtvus  20   0 3007m 247m  30m S    4 8.4  57:35.85 mythfrontend.re
>
> 1627 root      20   0  188m  51m  23m S    1 1.7   4:59.26 Xorg
>
>   595 root      20   0     0    0    0 S    0 0.0   0:20.11 jbd2/sda3-8
>
> 1419 mysql     20   0 2434m 152m 4212 S    0 5.2  75:42.51 mysqld
>
> 1462 root      20   0 32908  604  504 S    0 0.0   3:47.02 lircd
>
> 2705 nobody    20   0 16972  704  636 S    0 0.0   5:21.88 LCDd
>
> This in turn really slows the whole system down which is a problem 
> since it’s an all in one machine. Often it will drop out with 
> “irrecoverable recorder error” back to the recordings screen. The 
> recording doesn’t seem at affected though and I just start it again 
> and it works. 1000 minutes over 32 hours is a fair bit of CPU time 
> just for recordings. Doing something like moving a file from one disk 
> to another seems to make it even worse (but I get a get 60-70 
> MByte/sec transfer speeds)
>
> I only have a 1 instance of mythcommflag running at once but even 
> killing that one makes no difference. It’s a separate process anyway.
>
> This didn’t happen previously (was using Mythdora 12.25) on the same 
> tuners (same CPU but different motherboard).
>
> Otherwise it works quite well. Is this CPU usage likely to be MythTV 
> building the recordedseek table for the recordings that are running? 
> That’s about all I can think of. Is it normal now?
>
> Sorry about the length - tried to include as much detail as I can. I 
> don’t see much that is helpful in any logs.
>
> Any ideas? Is this just “normal”? With a few recordings it’s a bit 
> painful to use particularly when it keeps dropping out of playback. 
> The machine is not a powerhouse but should be easily capable of what 
> I’m doing.
>

That does sound a little high to me, but it could just be due to the 
fact that USB relies on the CPU for a lot (hence it's cheap).

As another data point for comparison, my system is sitting at around 
16-19% while recording 2 SD DVB-T programmes from a Nova T-500. This 
with a Celeron E3400 CPU, and writing to a mirrored pair of SATA drives.

thanks,
Alex

P.S. the Nova T-500 is actually a USB tuner, even though it looks like a 
PCI card - http://www.linuxtv.org/wiki/index.php/Hauppauge_WinTV-NOVA-T-500

-- 
Alex Tomlins
Email/Jabber: alex at tomlins.org.uk

There are two kinds of people in the world: those who finish what they started

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