Hi,
On Sep 1 2018 08:18, Guedes, Andre wrote:
I have another concern of buffering in a perspectives of delay of
task
scheduling.
The interval of task scheduling for this plugin is decided mainly by
the value of 'frames_per_pkt', given by users. In your documentation,
the value is 6[1]. Of cource this is an example but in this case the
interval is calculated as 125us at 48.0kHz. In my opinion, task
scheduling in Linux kernel brings deadline misses for the interval,
in most cases such as major Linux distribution on usual personal
computers. When considering about the fact that recent motherboards
implements Intel I210/220 series, it's better to care for the low-
level
realtime systems, in my opinion.
Agreed.
To mitigate this scheduling issue, a follow-up patchset will extend the
plugin to leverage the ETF qdisc [1] which will be available on next
kernel release. The idea is: instead of sending one AVTP packet at
every interval, the plugin will send several AVTP packets at once,
configuring their Tx time accordingly, so it can "sleep" for longer
periods of time. The goal is to ease on task scheduling by
offloading packet transmissions to the NIC.
A feature to queue packets with a transmission timing is mandatory for
applications to implement this kind of time-awareness packet
transmission protocol on general purpose operating system which has less
guarantees of its real-time capability.
In a case of IEC 61882-1/6 on IEEE 1394 bus, controllers of 1394 OHCI[1]
allows applications to queue several packets to corresponding
isochronous cycle. As a result, the controller can transfer each packet
at each isochronous cycle.
From my curiousity, would I ask you to explain about usage of the
ETF 'qdisk' for this kind of applications? How relevant hardware
guarantees timing of transmission for queued packets? Or network stack
on Linux kernel govern the transmission timing in the proposed
patchset?[2].
[1] 1394 Open Host Controller Interface Specification Release 1.1 (2000,
Promoters of the 1394 Open HCI)
http://download.microsoft.com/download/1/6/1/161ba512-40e2-4cc9-843a-923143f3456c/ohci_11.pdf
[2] [PATCH v2 net-next 00/14] Scheduled packet Transmission: ETF
https://marc.info/?l=linux-netdev&m=153065819412748
Thanks
Takashi Sakamoto
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