Re: [dpdk-dev] [PATCH RFC] librte_reorder: new reorder library

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Oct 08, 2014 at 04:07:28PM -0700, Matthew Hall wrote:
> On Wed, Oct 08, 2014 at 06:55:41PM -0400, Neil Horman wrote:
> >  I think because there is a possibility that multiple workers may be used for a
> > single tx queue.
> > 
> > Neil
> 
> OK, so, in my application packets are RX'ed to a predictable RX queue and core 
> using RSS.
> 
> Then you put them into a predictable TX queue for the same core, in the same 
> order they came in from the RX queue with RSS enabled.
> 
> So you've got a consistent-hashed subset of packets as input, being converted 
> to output in the same order.
> 
> Will it work, or not work? I'm just curious if my app is doing it wrong and I 
> need to fix it, or how this case should be handled in general...
> 
> Matthew.

Hi Matthew,

What you are doing will indeed work, and it's the way the vast majority of 
the sample apps are written. However, this will not always work for everyone 
else, sadly.

First off, with RSS, there are a number of limitations. On the 1G and 10G 
NICs RSS works only with IP traffic, and won't work in cases with other 
protocols or where IP is encapsulated in anything other than a single VLAN.  
Those cases need software load distribution. As well as this, you have very 
little control over where flows get put, as the separation into queues 
(which go to cores), is only done on the low seven bits. For applications 
which work with a small number of flows, e.g. where multiple flows are 
contained inside a single tunnel, you get a get a large flow imbalance, 
where you get far more traffic coming to one queue/core than to another.  
Again in this instance, software load balancing is needed.

Secondly, then, based off that, it is entirely possible when doing software 
load balancing to strictly process packets for a flow in order - and indeed 
this is what the existing packet distributor does. However, for certain 
types of flow where processing of packets for that flow can be done in 
parallel, forcing things to be done serially can slow things down. As well 
as this, there can sometimes be requirements for the load balancing between 
cores to be done as fairly as possible so that it is guaranteed that all 
cores have approx the same load, irrespective of the number of input flows.  
In these cases, having the option to blindly distribute traffic to cores and 
then reorder packets on TX is the best way to ensure even load distribution.  
It's not going to be for everyone, but it's good to have the option - and 
there are a number of people doing things this way already.

Lastly, there is also the assumption being made that all flows are 
independent, which again may not always be the case. If you need ordering 
across flows and to share load between cores then reordering on transmission 
is the only way to do things.

Hope this helps,

Regards,
/Bruce