Subject: [linux-audio-dev] APM killing low-latency performance on BX mainboard (we need a method to disable it dynamically)
From: Benno Senoner (sbenno_AT_gardena.net)
Date: ke marras 10 1999 - 17:49:41 EST
Hi,
I discovered a problem of APM which can ruin the performance of low-latency
(realtime) apps.
All tests performed using Ingo's 2.2.10-lowlatency-N6 patch:
On my new dual Celeron box with Gigabyte GA-6BX mainboard,
when enabling APM, ( Redhat comes with APM enabled by default),
there are regular IRQ losses when the APM driver polls the hardware.
( every second)
I discovered when I ran the latencytest benchmark:
With Ingo's patches, my old P133 delivered rock-solid (while stressed) 2.1ms
latencies.
While my Celeron366 (in UP mode, because SMP lowlatency performance
is not properly optimized on Ingo's patch), shows up regular 2ms peaks (at 1 sec
interval) when the box is completely idle.
With bigger audio fragment sizes the diagrams show clearly that there are losses
of IRQs when APM makes the BIOS calls.
When I disabled APM on my Celeron, the results were rock-solid without any
nasty peaks.
Note that disabling the power management in the BIOS and running an APM enabled
kernel did not help: APM remained active and the peaks at 1sec intervals were
still present.
Note that the same happens when I'm doing high freq timing tests using RTC.
Same peaks , same problems, and disabling APM solves the RTC problems too.
Now my question: in order to achieve best low-latency performance, we
need a way to disable APM on problematic boxes, but without recompiling
the kernel.
my proposals:
make a sysctl ( or is this already implemented) to disable APM at
runtime
or: make a kernel commandline boot option which allows to disable
APM at boottime, just like when the kernel detects that it is running on
a SMP box.
now the diagrams:
APM ENABLED:
4x128 audiobuffer
http://www.gardena.net/benno/linux/audio/apm/enabled/4x128.html
note the regular peaks at exactly 1 sec interval.
I was unableto get reliable playback with this buffersize on the Celeron+APM,
while the P133 gave me no problems.
4x8192 audiobuffer:
http://www.gardena.net/benno/linux/audio/apm/enabled/4x8192.html
note that this diagram shows that some IRQs of the soundcard get lost,
causing high latencies, and therefore potentially unreliabe playback when
using small buffersizes.
APM DISABLED: rock-solid results !
http://www.gardena.net/benno/linux/audio/apm/disabled/4x128.html
Note that in order to get really low-latencies all IDE devices must be DMA
enabled with unmaskirq and if possible 32bit access enabled:
I discovered a small (but easy to correct) problem:
by default ATAPI CDROM driver run with DMA , unmaskirq and 32bit I/O all three
disabled.
You can tune this using hdparm -d 1 -u 1 -c 1 /dev/hdc or so ,
but the problem is that you can't do this when there is no CD in the drive.
It would be nice a to add an option to hdparm and/or the ATAPI CDROM driver,
to force the mode changes, so that the distros could do this in the initscripts
by default.
For example I have a MASHITA DVD ATAPI as hdc,
and when mounting a CDROM and cp'ing the entire CD to the harddisk,
when the DVD/CDROM drive is in untuned mode, the latencies go
up to 20-70ms , but when tuning the CDROM drive with hdparm,
the latencies go down to a rock-solid 2ms !
comments, suggestions ?
regards,
Benno.
sbenno_AT_gardena.net
Linux low-latency benchmarks:
http://www.gardena.net/benno/linux/audio
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