tree d6855298c13b11deba6426b736d23d4aeeb1bdd3
parent c09403dcc5698abf214329fbbf3cf8dbb5558bea
author Konstantin Ozerkov <kozerkov@parallels.com> 1319641861 +0400
committer Takashi Iwai <tiwai@suse.de> 1320051808 +0100

ALSA: intel8x0: Improve performance in virtual environment

v3: detection code is x86 and KVM specific, hide it under ifdef
v2: add detection for virtual environments (KVM and Parallels)

This patch is intended to improve performance in virtualized environments
like Parallels Desktop or KVM/VirtualBox/QEMU (virtual ICH/AC97 audio).

I/O access is very time-expensive operation in virtual world: VCPU
can be rescheduled and in the worst case we get more than 10ms delay on
each I/O access.

In the virtual environment loop exit rule
(old_civ == current_civ && old_picb == current_picb) is never satisfied,
because old_picb is never the same as current_picb due to delay inspired
by reading current_civ. As a result loop ended by timeout and we get 10x
more I/O operations.

Experimental data from Prallels Desktop 7, RHEL6 guest (I/O ops per
second):

Original code:
In Port    Counter         Callback
   f014      41550         fffff00000179d00 ac97_bm_read_civ+0x000
   f018      41387         fffff0000017a580 ac97_bm_read_picb+0x000

With patch:
In Port    Counter         Callback
   f014       4090         fffff00000179d00 ac97_bm_read_civ+0x000
   f018       1964         fffff0000017a580 ac97_bm_read_picb+0x000

Signed-off-by: Konstantin Ozerkov <kozerkov@parallels.com>
Signed-off-by: Denis V. Lunev <den@openvz.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>