Subject: Re: qdisc spin lock

From: John Fastabend <[email-masked]>
Date: Thu, 31 Mar 2016 20:44:05 -0700

On 16-03-31 04:48 PM, Michael Ma wrote:
> I didn't really know that multiple qdiscs can be isolated using MQ so
> that each txq can be associated with a particular qdisc. Also we don't
> really have multiple interfaces...

MQ will assign a default qdisc to each txq and the default qdisc can
be changed to htb or any other qdisc of your choice.

> 
> With this MQ solution we'll still need to assign transmit queues to
> different classes by doing some math on the bandwidth limit if I
> understand correctly, which seems to be less convenient compared with
> a solution purely within HTB.
> 

Agreed.

> I assume that with this solution I can still share qdisc among
> multiple transmit queues - please let me know if this is not the case.

Nope sorry doesn't work that way unless you employ some sort of stacked
netdevice strategy which does start to get a bit complex. The basic hint
would be to stack some type of virtual netdev on top of a device and
run the htb qdisc there. Push traffic onto the netdev depending on the
class it belongs to. Its ugly yes.

Noting all that I posted an RFC patch some time back to allow writing
qdiscs that do not require taking the lock. I'll try to respin these
and submit them when net-next opens again. The next logical step is to
write a "better" HTB probably using a shared counter and dropping the
requirement that it be exact.

Sorry I didn't get a chance to look at the paper in your post so not
sure if they suggest something similar or not.

Thanks,
John

> 
> 2016-03-31 15:16 GMT-07:00 Cong Wang <[email-masked]>:
>> On Wed, Mar 30, 2016 at 12:20 AM, Michael Ma <[email-masked]> wrote:
>>> As far as I understand the design of TC is to simplify locking schema
>>> and minimize the work in __qdisc_run so that throughput won’t be
>>> affected, especially with large packets. However if the scenario is
>>> that multiple classes in the queueing discipline only have the shaping
>>> limit, there isn’t really a necessary correlation between different
>>> classes. The only synchronization point should be when the packet is
>>> dequeued from the qdisc queue and enqueued to the transmit queue of
>>> the device. My question is – is it worth investing on avoiding the
>>> locking contention by partitioning the queue/lock so that this
>>> scenario is addressed with relatively smaller latency?
>>
>> If your HTB classes don't share bandwidth, why do you still make them
>> under the same hierarchy? IOW, you can just isolate them either with some
>> other qdisc or just separated interfaces.