[alsa-devel] [PATCH - AAF PCM plugin 0/7] Follow-up improvements

[Takashi Iwai]

On Sat, 08 Dec 2018 02:55:43 +0100,
Andre Guedes wrote:
> 
> Hi all,
> 
> This patch series provides some follow-up improvements to the AVTP Audio
> Format (AAF) plugin. The highlight of this series is the implementation of
> a transmission offload mechanism which improves the plugin performance
> considerably. Details are provided in the following paragraphs.
> 
> Currently, the AVTPDU transmission interval is controlled by the AAF plugin, in
> software. At every timer expiration, the plugin transmits one AVTPDU.  This has
> some implications in terms of task scheduling. For instance, in class A
> streams, AVTPDUs are transmitted at every 125 us which means the application
> task should be scheduled-in at every 125 us. In a general purpose Linux system,
> such scheduling interval can be hard to cope with in the long-run.
> 
> To mitigate that issue, this series introduces a transmission offload mechanism
> that leverages the SO_TXTIME sockopt and ETF qdisc features recently introduced
> to kernel 4.19. Instead of sending one AVTPDU at every timer expiration, the
> plugin sends several AVTPDUs at once to the kernel, configuring their Tx time
> so the transmission interval is maintained. The kernel can then offload packet
> transmission to the hardware (if the network controller supports it), providing
> more transmission time accuracy.  This offloading mechanism enables the
> application to sleep for longer times, easing on task scheduling.  
> 
> To illustrate the improvements provided by this series, I ran a before-after
> experiment. The experiment setup consisted in 2 PCs with Intel i210 card
> connected back-to-back running an up-to-date Archlinux with kernel 4.19.4.  I
> ran 'aplay' for 5 minutes on one PC and captured the AVTPDUs on the other PC.
> The metric under evaluation is the transmission interval and it is measured by
> checking the 'time_delta' information from ethernet frames captured at the
> receiving side. If you're interested in reproducing the experiments, let me
> know and I can share my helper scripts.
> 
> The table below shows the experiment outcome for a Class A, stereo, 16-bit
> sample, 48 kHz stream. The unit is nanoseconds.
> 
>        |   Mean |   Stdev |     Min |     Max |   Range |
> -------+--------+---------+---------+---------+---------+
> Before | 125000 |    1154 |   75311 |  172144 |   96833 |
> After  | 125000 |      18 |  124960 |  125048 |      88 |
> 
> 
> Before this patchset, the transmission interval mean is equal to the optimal
> value (Class A stream -> 125 us interval), and it is kept the same after the
> patchset. However, the dispersion measurements had improved considerably,
> meaning the system is consistently transmitting AVTPDUs at the correct
> interval.
> 
> Finally, to help the review process, here follows a quick summary of the
> patches within this series:
> 	* PATCH 1: fixes a type in the plugin documentation.
> 	* PATCH 2: enables the user to configure the presentation time
> 	  tolerance.
> 	* PATCH 3-5: refactor the code in order to land the offload mechanism
> 	  code smoothly.
> 	* PATCH 6-7: implement the transmission offload mechanism.
> 
> This series can also be found in my alsa-plugins tree in github [1].
> 
> Regards,
> 
> Andre
> 
> [1] https://github.com/aguedes/alsa-plugins
> 
> Andre Guedes (7):
>   doc: Fix typo in AAF doc
>   aaf: Add presentation time tolerance
>   aaf: Refactor AVTPDU transmission routines
>   aaf: Refactor AVTPDU reception routines
>   aaf: Refactor timeout routines
>   aaf: Tx multiple AVTPDUs per media clock tick
>   aaf: AVTPDU transmission periodicity

Applied all patches now.  Thanks.


Takashi