Hi all,
I'm currently diving into the scheduler code from linux-generic to understand
how it works and try to write an optimize version for our HW.
Maybe I missed something, but I think there is a big fairness issue there.
Let me sum up what I get. We will stick with only one priority to make it
simpler.
We have QUEUES_PER_PRIO queues in the schduler that may contain commands.
Command are either:
- Poll a pktio
- Poll a queue
Both commands are pushed back to the queue if we didn't get all the pending
packet/buffer from them and need to be polled back.
Threads that call schedule do a round robin scan of all schedule queues
starting on the queue threadId % QUEUES_PER_PRIO, jumps to the next schedule
queue if the first schedule commands produced nothing. And stop on the first
schedule commands that produces packet.
Am I right up to that point?
Now let's assume I'm a user unaware of how this works. I have QUEUES_PER_PRIO
at 4. And running ODP with worker 4 threads.
I have created a bunch of queues for my classification so each of the schedule
queue as a 1 command to poll a queue in them.
For some reason I only want to call odp_schedule() from one thread.
Could be that the others are directly polling specific high priority queues.
When my thread enters odp_schedule, it starts with the schedule_queue = ( thId
% 4). Let's say 0.
Schedule queue 0 also contains the schedule command for my pktio.
Now let's see what happens
schedule()
check sched_queue 0
got sched_cmd to poll pktio
pktio_poll
dispatch 1 packet to each of my queues (this traffic is really well
balanced and regular ;) )
re enqueue the sched_cmd
continue
check sched_queue 1
got a sched_cmd to poll queue #1 ( which now has a packet)
return packet, requeue sched_cmd
schedule()
check sched_queue 0
got sched_cmd to poll a queue #0 (which also has a packet)
return packet, requeue sched_cmd
We are now in the exact same state as we were before the first call. Except
queue #2 and #3 now have 1 packet pending.
This should keep going on until we run out of packet in the pool...
So here are a few questions now:
1) Did I completely miss something and it actually works?
2) Why do linux-generic needs multiple queues per prio? Is it to reduce
contention on the sched_queue ?
Nicolas
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