Re: Periodic frame losses when recording to btrfs volume with OBS

[Sebastian Ochmann <ochmann@cs.uni-bonn.de>]

Hello,

I attached to the ffmpeg-mux process for a little while and pasted the 
result here:

https://pastebin.com/XHaMLX8z

Can you help me with interpreting this result? If you'd like me to run 
strace with specific options, please let me know. This is a level of 
debugging I'm not dealing with on a daily basis. :)

Best regards
Sebastian


On 22.01.2018 20:08, Chris Mason wrote:
> On 01/22/2018 01:33 PM, Sebastian Ochmann wrote:
> 
> [ skipping to the traces ;) ]
> 
>> 2866 ffmpeg-mux D
>> [<ffffffffc06251b1>] btrfs_start_ordered_extent+0x101/0x130 [btrfs]
>> [<ffffffffc061b0b0>] lock_and_cleanup_extent_if_need+0x340/0x380 [btrfs]
>> [<ffffffffc061c3c1>] __btrfs_buffered_write+0x261/0x740 [btrfs]
>> [<ffffffffc06202bf>] btrfs_file_write_iter+0x20f/0x650 [btrfs]
>> [<ffffffff9b235309>] __vfs_write+0xf9/0x170
>> [<ffffffff9b23551d>] vfs_write+0xad/0x1a0
>> [<ffffffff9b235762>] SyS_write+0x52/0xc0
>> [<ffffffff9b800083>] entry_SYSCALL_64_fastpath+0x1a/0x7d
>> [<ffffffffffffffff>] 0xffffffffffffffff
> 
> This is where we wait for writes that are already in flight before we're 
> allowed to redirty those pages in the file.  It'll happen when we either 
> overwrite a page in the file that we've already written, or when we're 
> trickling down writes slowly in non-4K aligned writes.
> 
> You can probably figure out pretty quickly which is the case by stracing 
> ffmpeg-mux.  Since lower dirty ratios made it happen more often for you, 
> my guess is the app is sending down unaligned writes.
> 
> -chris
> 
>