Low-latency CPU consumption

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Yesterday afternoon I tried to look a bit on where PulseAudio spends its 
time. In a low-latency scenario PulseAudio seems to be a bit CPU intensive.

The tests were done with perf analyzing PulseAudio git master + a 3.2 
kernel, and an AMD CPU that's ~3 years old or so. After starting 
PulseAudio I ran a 10 ms playback latency test during 30 seconds.

I'm still learning to master perf, but here are some initial results:

1) The top four functions (and more than half of the top 20) are all in 
the kernel, and a result from the ppoll call. It seems like we do about 
10 ppolls for every packet: three in the I/O thread and seven in the 
main thread. The majority of the main thread ppolls are due to the 
iochannel, i e talking to the client.
It looks like the iochannel is a bit inoptimal in its construction, 
requiring more ppolls than should be necessary. But I have to analyze 
this deeper. (I did a simple attempt to optimise it yesterday but it 
didn't work.)

2) Second is all the atomic operations we do, in handling our flists. 
There seem to be quite a penalty for doing memory fence operations, and 
there are plenty of those for just getting/putting something into an 
flist. Here it might also be possible to optimise things, maybe we can 
use a more efficient memory recycle mechanism for the most common 
workload or so.

3) Third is asking the kernel for the current hardware pointer. I have 
not looked into this deeper yet, but it's possible that we ask several 
times for the current hardware position where we should ask only once.

-- 
David Henningsson, Canonical Ltd.
https://launchpad.net/~diwic
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+   2.55%  lt-pulseaudio  [kernel.kallsyms]      [k] do_poll.isra.4                                                                                                       ?
+   2.34%  lt-pulseaudio  [kernel.kallsyms]      [k] fget_light                                                                                                           ?
+   2.14%  lt-pulseaudio  [kernel.kallsyms]      [k] _raw_spin_unlock_irqrestore                                                                                          ?
+   2.07%  lt-pulseaudio  [kernel.kallsyms]      [k] _raw_spin_lock_irqsave                                                                                               ?
+   1.85%  lt-pulseaudio  [snd_hda_intel]        [k] 0xc24                                                                                                                ?
+   1.55%  lt-pulseaudio  [kernel.kallsyms]      [k] find_busiest_group                                                                                                   ?
+   1.53%  lt-pulseaudio  [kernel.kallsyms]      [k] tg_load_down                                                                                                         ?
+   1.43%  lt-pulseaudio  libpulsecommon-3.0.so  [.] stack_pop                                                                                                            ?
+   1.37%  lt-pulseaudio  libpulsecommon-3.0.so  [.] pa_memblock_unref                                                                                                    ?
+   1.11%  lt-pulseaudio  libalsa-util.so        [.] thread_func                                                                                                          ?
+   1.06%  lt-pulseaudio  libpulsecommon-3.0.so  [.] stack_push                                                                                                           ?
+   1.05%  lt-pulseaudio  [kernel.kallsyms]      [k] unix_poll                                                                                                            ?
+   0.93%  lt-pulseaudio  [vdso]                 [.] 0x7fff2c1768c5                                                                                                       ?
+   0.89%  lt-pulseaudio  ld-2.15.so             [.] check_match.11236                                                                                                    ?
+   0.84%  lt-pulseaudio  libpulse.so.0.15.3     [.] pa_mainloop_dispatch                                                                                                 ?
+   0.80%  lt-pulseaudio  [kernel.kallsyms]      [k] __ticket_spin_lock                                                                                                   ?
+   0.79%  lt-pulseaudio  [kernel.kallsyms]      [k] ret_from_sys_call                                                                                                    ?
+   0.72%  lt-pulseaudio  [kernel.kallsyms]      [k] do_sys_poll                                                                                                          ?
+   0.68%  lt-pulseaudio  [kernel.kallsyms]      [k] fput                                                                                                                 ?
+   0.68%  lt-pulseaudio  [kernel.kallsyms]      [k] system_call                                                                                                          ?
+   0.68%  lt-pulseaudio  [kernel.kallsyms]      [k] update_cfs_load                                                                                                      ?
+   0.66%  lt-pulseaudio  [kernel.kallsyms]      [k] __schedule                                                                                                           ?
+   0.66%  lt-pulseaudio  libpulsecommon-3.0.so  [.] do_something                                                                                                         ?
+   0.65%  lt-pulseaudio  [kernel.kallsyms]      [k] fsnotify                                                                                                             ?
+   0.62%  lt-pulseaudio  libpulse.so.0.15.3     [.] pa_mainloop_prepare                                                                                                  ?
+   0.62%  lt-pulseaudio  [kernel.kallsyms]      [k] sysret_check                                                                                                         ?
+   0.60%  lt-pulseaudio  [kernel.kallsyms]      [k] __pollwait                                                                                                           ?
+   0.58%  lt-pulseaudio  [kernel.kallsyms]      [k] copy_user_generic_string                                                                                             ?
+   0.57%  lt-pulseaudio  libpulsecommon-3.0.so  [.] pa_flist_pop                                                                                                         ?
+   0.55%  lt-pulseaudio  [kernel.kallsyms]      [k] eventfd_poll                                                                                                         ?
+   0.53%  lt-pulseaudio  [kernel.kallsyms]      [k] native_write_msr_safe                                                                                                ?
+   0.52%  lt-pulseaudio  [kernel.kallsyms]      [k] find_next_bit                                                                                                        ?
+   0.51%  lt-pulseaudio  libpulsecore-3.0.so    [.] pa_rtpoll_run                                                                                                        ?


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