[ovs-dev] [PATCH RESEND ovs v3 4/4] dpif: Don't set "burst_size" to "rate" if not specified.

[Jean Tourrilhes]

On Tue, Mar 30, 2021 at 02:27:11PM -0700, Ben Pfaff wrote:
> On Tue, Mar 30, 2021 at 11:16:48PM +0200, Ilya Maximets wrote:
> > 
> > OpenFlow spec is a bit loose in definition of what should
> > be behavior if burst is not set:
> > """
> > If the flag OFPMF_BURST is not set the burst_size values from meter
> > bands are ignored, and if the meter implementation uses a burst value,
> > this burst value must be set to an implementation defined optimal value.
> > """
> > 
> > In our case, historically, "implementation defined optimal value" was
> > value equal to rate.  I have no idea why, but it's hard to argue with
> > it since the spec gives a great freedom to choose.
> > 
> > Actually, the "burst" itself as a term makes very little sense to me.
> > It's defined by the spec as:
> > """
> > It defines the granularity of the meter band, for all packet or byte
> > bursts whose length is greater than burst value, the meter rate will
> > always be strictly enforced.
> > """
> > 
> > But what is the burst?  How the implementation should define which
> > packets are in the burst and which are from the next one?
> > 
> > Current implementation just assumes that bursts are measured per second.
> > But the rate is measured per second too.  So, burst and rate is
> > essentially the same thing and implementations just sums them together
> > to get the bucket size.  So, I do not understand why "burst" and
> > "burst_size" exist at all.  Why not just set the rate a bit higher?
> > 
> > Ben, can you shed some light on this?  What was the original idea
> > behind the meter burst?  Or maybe I'm missing something?

	I don't understand how you can confuse a rate and a size. The
OpenFlow spec clearly says it's in kilobits or packets (not per
seconds).
	A basic token bucket has only two parameters, the commited
rate and the burst size (i.e. maximum number of tokens in the
bucket). The spec reflect that in a generic way to avoid mandating an
implementation.
	Burst rate is only defined for more fancy rate limiters, such
as two colors rate limiters. In this case, you also have two burst
size, one for each token bucket. The OpenFlow spec does not support
those extra parameters (as of version 1.5.1).
	For Linux 'police' filter : rate == rate ; burst_size == burst
	For Linux 'htb' qdisc : rate == rate ; burst_size == burst ;
ceil and cburst are not supported.

> I wasn't really involved in the design of meters.  I saw them as a
> feature of hardware switches that was not very relevant to software
> switches.  I guess I was wrong.
> 
> I think that Jean Tourillhes was the primary architect of meters in
> OpenFlow.  I am adding him to this thread.  I hope that he can help.

	Have fun...

	Jean