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

Christian Hack christian at hack.id.au
Sun Jul 14 10:57:05 UTC 2013


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"

 

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.

 

Thanks

Christian

 

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