[alsa-devel] [PATCH 1/1] intel8x0: Improve performance in virtual environment

[Takashi Iwai]

At Thu, 20 Oct 2011 19:02:50 +0400,
Konstantin Ozerkov wrote:
> 
> This patch intended to improve performance in virtualized environments
> like Parallels Desktop or 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 got more than 10ms delay on
> each I/O access.
> 
> In the original code normal loop exit rule
> (old_civ == current_civ && old_picb == current_picb) was never satisfied,
> because old_picb was never the same as current_picb due delay inspired by
> reading current_civ. As a result loop ended by timeout and we got 10x more
> I/O operations.

You mean in the virtual environment, right?  On the real machines,
this condition can be satisfied normally because the read is much
faster than the sample rate.  This check is needed to avoid the broken
behavior around the boundary.

> To prevent glitch on buffer swapping the rule (old_civ == current_civ) is
> enough.

Not for real (thus less logical) machines, unfortunately.  Both checks
were needed in the real tests.

I don't mind to implement some bypass if the driver knows that it's
running in a VM.  But, removing blindly would cause definitely a
regression.


thanks,

Takashi


> 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 at parallels.com>
> Signed-off-by: Denis V. Lunev <den at parallels.com>
> 
> diff --git a/pci/intel8x0.c b/pci/intel8x0.c
> index 6a5b387..bad4a65 100644
> --- a/pci/intel8x0.c
> +++ b/pci/intel8x0.c
> @@ -1065,8 +1065,7 @@ static snd_pcm_uframes_t snd_intel8x0_pcm_pointer(struct snd_pcm_substream *subs
>  			udelay(10);
>  			continue;
>  		}
> -		if (civ == igetbyte(chip, ichdev->reg_offset + ICH_REG_OFF_CIV) &&
> -		    ptr1 == igetword(chip, ichdev->reg_offset + ichdev->roff_picb))
> +		if (civ == igetbyte(chip, ichdev->reg_offset + ICH_REG_OFF_CIV))
>  			break;
>  	} while (timeout--);
>  	ptr = ichdev->last_pos;
> -- 
> 1.7.7
>