[ovs-dev] [PATCH v5] tunneling: Avoid recirculation on datapath by computing the recirculate actions at translate time.

[Chandran, Sugesh]

Regards
_Sugesh


> -----Original Message-----
> From: Zoltán Balogh [mailto:zoltan.balogh at ericsson.com]
> Sent: Friday, May 26, 2017 3:01 PM
> To: Joe Stringer <joe at ovn.org>
> Cc: Chandran, Sugesh <sugesh.chandran at intel.com>; Andy Zhou
> <azhou at ovn.org>; William Tu <u9012063 at gmail.com>;
> gvrose8192 at gmail.com; ovs dev <dev at openvswitch.org>; Ben Pfaff
> <blp at ovn.org>; Jan Scheurich <jan.scheurich at ericsson.com>; Gray, Mark D
> <mark.d.gray at intel.com>
> Subject: RE: [ovs-dev] [PATCH v5] tunneling: Avoid recirculation on datapath
> by computing the recirculate actions at translate time.
> 
> 
> Hi Joe,
> 
> > Backing up a bit for context, the stats attribution goes roughly like this:
> > * First upcall, handler thread calls through the translate code with a
> > packet. The resubmit_stats are derived from that packet. This goes
> > through xlate_actions().
> > * First dump of flow from revalidator thread fetches the flow and runs
> > the same xlate_actions() with whatever stats it has (may be zero).
> > This time, whenever stats attribution or side effects occur, an
> > xlate_cache entry is generated.
> > * Second and subsequent dumps of flows fetches the flow and shortcuts
> > the xlate_actions() by using the xlate_cache instead - ie a call to
> > xlate_push_stats().
> >
> > So, in the same place where the resubmit_stats is manipulated, you
> > would also need to generate a new XC entry which would manipulate the
> > stats - this would be a 'side-effect'. I'd imagine that prior to the
> > full output translation there would be a XC_TRUNCATE(truncated_size)
> > then afterwards there would be an XC_TRUNCATE_RESET(). Or it could be
> > just XC_SET_SIZE(...) where 0 is reset and non-zero is a truncate
> > size. In the implementation/execution in xlate_push_stats() when
> > performing XC_TRUNCATE you would need to store the original push_stats
> > size somewhere, then calculate a new 'n_bytes' based on the number of
> > packets and existing bytes*. For XC_TRUNCATE_RESET(), it would restore
> > the original push_stats size.
> 
>  Thank you for the explanation.
> 
> > * Hmm, I'm not sure the calculation will be 100% here. Let's say there
> > were 3 packets hit the flow, 50B, 200B, 300B. If output(max_len=100)
> > was executed, then we don't know how many of the packets were
> > truncated. The maximum number of bytes that could be transmitted is
> > 300, but the actual number was 250. We could divide the n_bytes by
> > n_packets, subtract the max_len and then multiply back up by the
> > number of packets, which works for this case assuming floating point
> > arithmetic but is slightly off if using integer math..
> 
> I don't think, that would be the proper way of calculating n_bytes. Let's say
> we have 3 packets with 50B, 200B, 200B and max_len=100. The output should
> be 50 + 100 + 100 = 250B.
> Following the instructions above we will get
> [(50 + 200 + 200) / 3 - 100 ] * 3 = [450 / 3 - 100 ] * 3 = 50 * 3 = 150B
> 
> Any other idea how to calculate the truncated size with xlate cache?
> Or maybe I did not understand your calculation.
[Sugesh] Since we have this issue with the trunc action,
How about limit the combine action only for those tunnels that don’t have any post trunc action.
If there is a trunc action, Create two separate rules normally as now.
Is there any other action that would be considered as exception like this?

> 
> There is one more thing to be taken into consideration. By adding a tunnel
> header, the size of packets increases as well. But that's a constant value for
> each packet, easier to calculate with it.
> 
> Best regards,
> Zoltan