Manoj and Luiz -
On Tue, 31 Jul 2012, Luiz Augusto von Dentz wrote:
Hi Manoj,
On Tue, Jul 31, 2012 at 2:30 PM, Manoj Sharma <ursmanoj@xxxxxxxxx> wrote:
Hi Luiz,
On 7/30/12, Luiz Augusto von Dentz <luiz.dentz@xxxxxxxxx> wrote:
Hi Manoj,
On Mon, Jul 30, 2012 at 9:30 AM, Manoj Sharma <ursmanoj@xxxxxxxxx> wrote:
One problem which I have faced using SO_PRIORITY is explained below.
Suppose we have 2 links A & B and link A has higher priority than link
B. And outgoing data transfer is active on both links. Now if device
on link A goes far, there would be lot of failures and number of
re-transmissions would increase for link A. Consequently at any time
host would have significant number of packets for link A, getting
accumulated due to poor quality of link.But since link A packets have
higher priority, link B packets would suffer infinitely as long as
link A packet queue in host is non-empty. Thus link B protocols may
fail due to timers expiring and finally disconnection at upper layers.
There is a mechanism to avoid starvation, also apparently you didn't
study the code since the priority is per L2CAP channel not per link so
we are able to prioritize per profile.
I would check how starvation is avoided. But for your information I
did observe starvation practically. And I know that priority is per
L2CAP. I mentioned links based on assumption that AVDTP and OBEX are
connected with different devices. Hence priority would result into
priority of connections in such case ;).
There is no such thing of prioritize a connection, the algorithm used
always check every channel of each connection and prioritize the
channel. Maybe you are confusing what some controllers do, the
controller has no idea what L2CAP channel has been configured it only
knows about the ACL connections.
The current starvation avoidance algorithm works well as long as there
is data queued for all of the L2CAP channels and the controller is
sending packets regularly. It does seem to break down when the
controller queue gets stuck (see below).
Second problem:
We have two links similar to above scenario. Say link A is being used
by AVDTP and link B is being used by OBEX. Host can come across a
situation where all controller buffers are used by OBEX and AVDTP is
waiting for a free buffer. Now due to some reason (e.g. distance) OBEX
link B goes weak. This results into delay in transmission of OBEX
packets already held by controller and consequently AVDTP packets also
get delayed which causes glitches in music streaming and user
experience goes bad.
That is exactly what SO_PRIORITY has fixed, by setting SO_PRIORITY you
prioritize AVDTP stream over OBEX which means AVDTP can use a bigger
part of the bandwidth while OBEX uses the remaining.
I disagree. Please try to understand the situation I explained again.
There can be a scenario when host has only OBEX packets and no AVDTP,
here irrespective of which channel has what priority OBEX may consume
all ACL credits. At the same moment OBEX link goes weak (e.g.due to
distance), this would delay the transmission of all OBEX packets held
by controller. In the mean time, AVDTP packets reach Bluez but since
there are no credits left, host would have to delay transmission of
AVDTP until a OBEX packet is transferred and an NOCP is received. This
would definitely cause a glitch on AVDTP streaming and end user
experience would go bad. By reserving credits for AVDTP channel, we
ensure that OBEX packets doesnt eat up all credits while AVDTP packets
were absent.
Without the use of guaranteed channels you cannot really guarantee
anything, besides this would throttle OBEX transfer even when there is
nothing streaming on AVDTP which I don't thing is acceptable. Also Ive
never experience such a problem, you can start streaming while
transferring something and that never produced any artifacts in the
headsets I have, the only problem we have right now is paging another
device while AVDTP stream is active may cause some audio glitches and
even that could be avoided by tuning paging parameters while there is
a high priority channel active.
Btw, there is some lack of connection to the code, an OBEX packet
could be quite big but that is not transmitted as it is, it is
actually fragmented into L2CAP and then HCI frames, the HCI frames is
the one being sent to the controller, the moment the AVDTP socket
start producing another socket may be using some/all of the controller
buffers e.g. 8:1021 that is at most 8K bytes of latency to startup the
stream, in fact it is pretty common to audio to have some latency.
It's true that the amount of latency due to buffering in the
controller is minimal, and would not in itself cause streaming issues
*if* the controller is able to consume packets from the buffer.
I do see where a bad situation arises when the OBEX connection is
stalled and only queued OBEX data is available to the host stack HCI
scheduler at that instant. In that case, the controller queue could
be completely consumed by data for the stalled channel no matter what
the priorities are. This could even happen when audio data is passed
to the socket at exactly the right time.
If you're using OBEX-over-L2CAP, this could be partially addressed by
setting a flush timeout. However, it would still be possible to fill
the buffers with OBEX packets because non-flushable ERTM s-frames
would accumulate in the controller buffer.
For traditional OBEX-over-RFCOMM, an OBEX packet sizes that are
smaller than the controller buffer could help. This is a tradeoff
against throughput. It could work to send smaller OBEX packets when
an AVDTP stream is active, even if a larger OBEX MTU was negotiated.
It would be a big help if Manoj could post kernel logs showing us how
the HCI scheduler is actually behaving in the problem case.
While I'm convinced that a problem exists here, I think it can be
addressed using existing interfaces instead of adding a new one. For
example, it may be reasonable to not fully utilize the controller
buffer with data for just one ACL, or to use priority when looking at
controller buffer utilization. Maybe an ACL could use all but one
slot in the controller buffer, maybe it could only use half if there
are multiple ACLs open. I don't think it would throttle throughput
unless the system was so heavily loaded that it couldn't respond to
number-of-completed-packets in time at BR/EDR data rates, and in that
case there are bigger problems. It's pretty easy to test with
hdev->acl_pkts set to different values to see if using less of the
buffer impacts throughput.
Right now, one stalled ACL disrupts all data flow through the
controller. And that seems like a problem worth solving.
The credit based algorithmic actually complicates more than solves the
problems here because it should actually fail if there is no enough
bandwidth as requested, so we would actually need to query how much
credits are available, also any type of bandwidth reservation might be
overkill with things like variable bit rate where you actually need to
know what is maximum possible bandwidth you need to reserve before
hand and that credits cannot be reserved by anyone else.
I agree, but we can provide a mechanism to allow only one channel to
reserve bandwidth. In most cases it would be AVDTP streaming channel.
Reserving at least one credit would allow preventing cases where
non-AVDTP channel eats all credits due to unavailability of AVDTP
packets. Please mind that since OBEX packets would be reaching bluez
much faster than AVDTP, such situation may arise very easily.
That raises my suspicious that you are not really testing against
PulseAudio and obexd, PA should be sending packets much faster than
obexd since its IO threads are realtime so it will most likely have
higher priority. Also the latency of OBEX packets are much greater as
each packet is normally 32k-64k compared to AVDTP stream which send
each packet individually (~700 bytes depending on the MTU).
These are the basic problems which I have faced and hence felt
necessity of a similar but different mechanism and came up with this
solution. This solution fixes both of the problems explained above.
Based on the explanation above your suggestion is required further.
Could you please show us what system did you find this problem? We
could possible help you trying to figure out what is going wrong,
please note that SO_PRIORITY support was introduced in 3.0 and some
system don't actually use it, in fact so far I think only PulseAudio
make use of it.
Yes, but we forced Bluez AVDTP to use SO_PRIORITY on our system and
faced the starvation problem explained above. Though I am going to
study the priority patch again.
Im afraid the problem is not SO_PRIORITY but your audio subsystem
cannot keep up the socket buffer non-empty that would avoid OBEX
taking too much bandwidth, but again that is pretty strange as you
should be written much more frequently to the AVDTP socket to keep the
latency of the audio constant.
I agree that SO_PRIORITY is not the problem, but I don't think this
can be fixed at the audio subsystem level either.
Regards,
--
Mat Martineau
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum
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