19.09.2014 02:07, Alexander E. Patrakov wrote:
> 18.09.2014 13:13, David Henningsson wrote:
>> Calling snd_pcm_avail/delay causes a syscall to the kernel, which
>> communicates with the audio hardware, and can therefore be expensive
>> on some cards.
>>
>> By only updating this value after a sleep and after unusual events,
>> we can reduce calls to update the hardware pointer.
>> In particular, if a write goes well, we will now assume that the
>> buffer has been filled up, instead of re-asking the hardware that
>> this is actually the case.
>
> The patch causes an assertion failure on my laptop.
>
> E: [alsa-sink-ALC275 Analog] alsa-sink.c: Assertion 'frames > 0'
> failed at modules/alsa/alsa-sink.c:651, function mmap_write(). Aborting.
>
This is somehow related to autosuspending the sink. I have worked around
that by not letting it suspend.
I have tested the performance implications of the patch on my laptop.
The methodology is:
1. Force the frequency of all CPU cores to 800 MHz
2. In one terminal, run: pulseaudio -k ; perf record ./src/pulseaudio -v
3. In another terminal, run: aplay -f cd -d 60 --period-size=44100
--buffer-size=88200 /dev/zero ; pulseaudio -k
The test was also repeated with --period-size=441 --buffer-size=882.
In both cases, the "perf report" output is almost identical with and
without the patch. For brevity, I omit everything below 1%.
For 1s period, without the patch:
# Overhead Command Shared
Object Symbol
# ........ .......... ..............................
...............................................
#
14.44% pulseaudio libasound.so.2.0.0 [.]
softvol_convert_stereo_vol
8.51% pulseaudio [kernel.kallsyms] [k] __lock_acquire
5.12% pulseaudio libasound.so.2.0.0 [.]
snd_pcm_format_little_endian
2.87% pulseaudio [kernel.kallsyms] [k] lock_release
1.74% pulseaudio [kernel.kallsyms] [k] lock_acquire
1.56% pulseaudio [kernel.kallsyms] [k] mark_lock
1.53% pulseaudio ld-2.19.so [.] strcmp
1.51% pulseaudio libasound.so.2.0.0 [.]
snd_pcm_format_cpu_endian
1.39% pulseaudio ld-2.19.so [.] do_lookup_x
1.25% pulseaudio libasound.so.2.0.0 [.]
snd_pcm_format_little_endian at plt
1.10% pulseaudio [kernel.kallsyms] [k]
trace_hardirqs_on_caller
1.03% pulseaudio libasound.so.2.0.0 [.]
snd_pcm_format_cpu_endian at plt
(don't panic! softvol_convert_stereo_vol doesn't eat > 14% of the CPU
here, it eats 13% of all busy cycles. I.e. other applications that
happened to be running on this mostly-idle laptop and other PulseAudio
functions ate 7x more CPU time.)
For 1s period, with the patch:
# Overhead Command Shared
Object Symbol
# ........ .......... .....................
...............................................
#
13.67% pulseaudio libasound.so.2.0.0 [.]
softvol_convert_stereo_vol
7.74% pulseaudio [kernel.kallsyms] [k] __lock_acquire
5.32% pulseaudio libasound.so.2.0.0 [.]
snd_pcm_format_little_endian
3.41% pulseaudio [kernel.kallsyms] [k] lock_release
2.48% pulseaudio [kernel.kallsyms] [k] lock_acquire
2.04% pulseaudio ld-2.19.so [.] do_lookup_x
1.64% pulseaudio [kernel.kallsyms] [k] mark_lock
1.39% pulseaudio [kernel.kallsyms] [k]
trace_hardirqs_on_caller
1.33% pulseaudio libasound.so.2.0.0 [.]
snd_pcm_format_cpu_endian at plt
1.23% pulseaudio [kernel.kallsyms] [k]
trace_hardirqs_off_caller
1.02% pulseaudio ld-2.19.so [.] strcmp
For 0.01s period, without the patch:
# Overhead Command Shared
Object Symbol
# ........ .......... ................................
.....................................................
#
13.12% pulseaudio [kernel.kallsyms] [k]
__lock_acquire
4.23% pulseaudio [kernel.kallsyms] [k] lock_release
2.78% pulseaudio [kernel.kallsyms] [k] lock_acquire
1.93% pulseaudio [kernel.kallsyms] [k] __fget
1.74% pulseaudio [kernel.kallsyms] [k] do_sys_poll
1.61% pulseaudio [kernel.kallsyms] [k] mark_lock
1.47% pulseaudio libasound.so.2.0.0 [.]
softvol_convert_stereo_vol
1.27% pulseaudio [kernel.kallsyms] [k]
trace_hardirqs_on_caller
For 0.01s period, with the patch:
# Overhead Command Shared
Object Symbol
# ........ .......... ..........................
...............................................
#
13.03% pulseaudio [kernel.kallsyms] [k] __lock_acquire
4.18% pulseaudio [kernel.kallsyms] [k] lock_release
2.78% pulseaudio [kernel.kallsyms] [k] lock_acquire
1.81% pulseaudio [kernel.kallsyms] [k] __fget
1.74% pulseaudio [kernel.kallsyms] [k] do_sys_poll
1.51% pulseaudio [kernel.kallsyms] [k] mark_lock
1.48% pulseaudio libasound.so.2.0.0 [.]
softvol_convert_stereo_vol
1.32% pulseaudio [kernel.kallsyms] [k]
trace_hardirqs_on_caller
I.e. the effect of the patch is very minor.
--
Alexander E. Patrakov
