[mythtv-users] Scheduler interrupts playback

Paul Warren pdw at ex-parrot.com
Thu Mar 1 10:18:17 UTC 2012


Thanks for the reply.

On 29/02/2012 22:10, Raymond Wagner wrote:
>> Backend is 800MHz AMD Athlon(tm) II Neo N36L Dual-Core.
>> MySQL is on a single SATA disk.  video files are on a software RAID 10
>> array across 4 other SATA disks.
>> FS on both is ext4.
> Unless you've underclocked it further, that chip runs at 1.3GHz.  Maybe
> you're looking at the C&Q speed or memory clock.

I've not underclocked it, but I was going based on this line in 
/proc/cpuinfo: cpu MHz:800.000.  Out of interest, is there a more 
reliable way to determine chip speed?

> The issue you're seeing is likely a mix of lower end CPU, and the EXT4
> barriers behavior that has been complained about on the mailing list for
> around a year now.  EXT4 enables barriers by default, which means writes
> flushed to disk must reach the platter, rather than simply hitting
> another stage of cache.  MySQL flushes its writes for atomicity, so each
> time you store a new seek point, update a filesize, or one of a number
> of other things that results in several inserts or updates per second
> per recording, MySQL has to wait for your disk.
> Normally, MySQL would have no trouble handling this kind of load.
> However, if the scheduler is running full out on the database at the
> same time, everything starts to get backed up, resulting in playback
> pauses, and delayed startup of recordings.

So the bit I don't understand is why MySQL becoming blocked affects 
playback, given that the DB and the playback files are on different disks.


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