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

[Zoltán Balogh]

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.

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