Summary of osmo-pcu failures/defects

Holger Hans Peter Freyther hfreyther at sysmocom.de
Mon Nov 25 23:18:03 UTC 2013


On Mon, Nov 25, 2013 at 09:21:08AM +0100, Holger Hans Peter Freyther wrote:

> As I have already pointed out. Some control blocks can starve. I don't
> know if it does but it is where I will continue to have a look.


Here is the starvation theory for the ping:

We have a LLC frame (or many queued up)... we schedule the polls and
at the final_ack indicate we decide to re-use the TBF. This means that
we will schedule another PACKET DOWNLINK ASSIGNMENT. But at the same
time we either want to honor the "rh->si" or want to schedule the ACK
due SEND_ACK_AFTER_FRAMES.

So we more or less want to send "PACKET DOWNLINK ASSIGNMENT" and the
"PACKET UPLINK ACK" at the same time (with more DL tbfs/traffic this
is getting more likely) but currently we will always prefer the PACKET
DOWNLINK ASSIGNMENT. This means that the uplink will starve (e.g. the
window stalled, rh->si means that the uplink will starve (the window
will stall, rh->si being set, etc).


Fairness improvements:

* put ul_ass_tbf, dl_ass_tbf, ul_ack_tbf in an array and store the
  last index in the PDCH and iterate over it.
* move the TBF to the front of the ul_tbfs and dl_tbfs list so that
  it is not selected again.


-- 
- Holger Freyther <hfreyther at sysmocom.de>       http://www.sysmocom.de/
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