Re: lantency scheduling benchmarks of audio playing tasks during high disk I/O

Benno Senoner (sbenno@gardena.net)
Mon, 21 Jun 1999 15:33:11 +0200


On Mon, 21 Jun 1999, Andrea Arcangeli wrote:
> On Mon, 21 Jun 1999, Benno Senoner wrote:
>
> >--------------+-------------+---------------+-------------+--------------+
> >buffer size |proc (top) | disk write | disk copy | disk read |
> >--------------+-------------+---------------+-------------+--------------+
> >2x1024(11.6ms)| 15.1ms (307)| 9610.0ms (221)| 20.5ms ( 2)| 867.3ms ( 10)|
> ^^^^^^ ^^^^^
> Could you explain the meaning of the two fields? Which bench are you using
> to generate the numbers?

2 fragments of 1024 bytes = 11.6ms

15.1ms is the maximum time difference between
2 consecutive write() to /dev/dsp and 307 times the time diff was
greater than 11.6ms , therefore there were 307 sound drop outs.

I use my usual bench ( http://www.gardena.net/benno/linux/latencytest0.3.tgz )

the tableis generated by:
./runalltests filesize ( for filesize (in bytes) use a size of at least 2
times your RAM to avoid caching.

>
> >the andrea patch behaves very well during disk copy (cp file1 file2) operations,
> >but on write only , or read only operations , it gives extremely high
> >latencies.
>
> This sounds to me as a bit weird. A copy always imply a read and a
> write... so it should be the slower one. This made me to think that the
> numbers got fooled by a far different working set across benchmarks run.
> Is this possible?

Mybe the cause is that , issuing read and writes in sequence , cause your kernel
to respond in more smoothly, because massive writes get interrupted by the read.

>
> If not I would like if you could try without the wait_for_IO hack.
>

ok, I will net you know, but I think your patches are not the definitive cure
of the high-latency problems, due to the kernel locking issue.

regards,
Benno.

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