#10045 HIGH 1.5-sof: XO-1.5 Record audio/video are out of sync with each other

Tiago Marques tiagomnm at gmail.com
Mon May 10 12:33:07 EDT 2010


On Mon, May 10, 2010 at 7:37 AM, James Cameron <quozl at laptop.org> wrote:

> On Sat, May 08, 2010 at 05:24:00PM +0000, Tiago Marques wrote:
> > I was just looking into this and the issue with SD cards' random write
> > performance intrigued me in this particular case.  Isn't recording
> > being performed sequentially, hence no lag problem?
>
> At the moment, assuming an application that opens a file, and writes to
> it at _any_ speed, occasionally the write will block for a huge amount
> of time.
>
> As an example, consider this shell script:
>
> #!/bin/sh
> while true; do
>        echo ok
>        sleep 0.01
> done
>
> And then this command to execute it:
>
> strace -o /tmp/k -e write -T /tmp/j > /root/file
>
> And then after several minutes or hours of run the strace file is
> analysed:
>
> grep write k|cut -f2 -d'<'|cut -f1 -d'>'|sort -rn|more
>
> One will find several instances of write(2) syscalls taking huge
> amounts of time:
>
> 0.406725
> 0.347159
>
> Now this is surprising given that the I/O rate is trivial, and that
> there is ample memory for buffering the writes.
>

Indeed. But that's the price paid for non-cached 0.04KB/s random write
media. I've witnessed some block as much as 2s under heavy I/O.
Also, I've been experimenting with I/O schedulers and they seem to help,
although I haven't done any measurement, just using the XO. I should try
your test. Someone tested the schedulers and they do make a difference:

http://www.ocztechnologyforum.com/forum/showthread.php?53573-NILFS2-Linux-File-System...-Have-you-tried-it&p=365930&viewfull=1#post365930

I only tried noop because deadline is not built in the Fedora XO kernel, but
it has been also suggested as an alternative, possibly better than noop:

nilfs scheduler noop 0:15min
nilfs scheduler cfq 0:27min
btrfs scheduler cfq 0:35min
ext2 scheduler cfq 1:38min
ext2 scheduler noop 1:29min


>
> If the test is repeated with the writes directed to tmpfs, there are no
> such delays.
>
> If the test is repeated at a much faster I/O rate using dd(1), ensuring
> that ample free memory is available to buffer the writes, then one still
> gets huge delays.  Consider this test sequence:
>
> a.  drop all caches,
>
> b.  verify free memory,
>
> c.  write 262144000 bytes in 8192 byte blocks,
>
> d.  verify cache is used,
>
> e.  look at the distribution of the slowest writes.
>
> [root at xo-a7-2a-5a tmp]# echo 1 > /proc/sys/vm/drop_caches
> [root at xo-a7-2a-5a tmp]# echo 2 > /proc/sys/vm/drop_caches
> [root at xo-a7-2a-5a tmp]# echo 3 > /proc/sys/vm/drop_caches
> [root at xo-a7-2a-5a tmp]# free
>             total       used       free     shared    buffers     cached
> Mem:        949256      96328     852928          0         88      30304
> -/+ buffers/cache:      65936     883320
> Swap:            0          0          0
> [root at xo-a7-2a-5a tmp]# time strace -o /tmp/k -e write -T dd if=/dev/zero
> of=/root/file bs=8192 count=32000
> 32000+0 records in
> 32000+0 records out
> 262144000 bytes (262 MB) copied, 59.8669 s, 4.4 MB/s
>
> real    0m59.929s
> user    0m1.960s
> sys     0m5.350s
> [root at xo-a7-2a-5a tmp]# free
>             total       used       free     shared    buffers     cached
> Mem:        949256     361192     588064          0        672     287568
> -/+ buffers/cache:      72952     876304
> Swap:            0          0          0
> [root at xo-a7-2a-5a tmp]# grep write k|cut -f2 -d'<'|cut -f1 -d'>'|sort
> -rn|more
> 11.648413
> 8.020875
> 4.299857
> 3.282466
> 2.960739
> 2.151048
> 2.147674
> 2.090824
> 1.972840
> 1.740923
> 1.721511
> 1.674147
> 1.652831
> 1.538634
> 1.430747
> 1.417739
> 1.000022
> 0.961700
> 0.847079
> 0.009241
> 0.009202
> 0.005332
>
> I've no explanation for this behaviour.
>
>
hmmm? But this is to the tmpfs???


> I don't think any different filesystem will affect the issue, but it
> would probably be a good thing to try.  Let us know how it goes!
>
>
As you can see by the above tests, there is a good difference, it probably
won't mitigate the problem but should make it more bearable. Must recompile
some kernels, perhaps I'll have something by the end of the week.

Best regards,
Tiago


> --
> James Cameron
> http://quozl.linux.org.au/
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> Devel at lists.laptop.org
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>
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