Re: [dpdk-dev] [PATCH 11/13] mbuf: move l2_len and l3_len to second cache line

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

On Thu, Sep 04, 2014 at 05:00:12PM +0600, Yerden Zhumabekov wrote:
> I get your point. I've also read throught the code of various PMDs and
> have found no indication of setting l2_len/l3_len fields as well.
> 
> As for testing, we'd be happy to test the patchset but we are now in
> process of building our testing facilities so we are not ready to
> provide enough workload for the hardware/software. I was also wondering
> if anyone has run some test and can provide some numbers on that matter.
> 
> Personally, I don't think frag/reassemly app is a perfect example for
> evaluating 2nd cache line performance penalty. The offsets to L3 and L4
> headers need to be calculated for all TCP/IP traffic and fragmented
> traffic is not representative in this case. Maybe it would be better to
> write an app which calculates these offsets for different set of mbufs
> and provides some stats. For example, l2fwd/l3fwd + additional l2_len
> and l3_len calculation.
> 
> And I'm also figuring out how to rewrite our app/libs (prefetch etc) to
> reflect the future changes in mbuf, hence my concerns :)
>
Just a final point on this. Note that the second cache line is always being
read by the TX leg of the code to free back mbufs to their mbuf pool post-
transmit. The overall fast-path RX+TX benchmarks show no performance
degradation due to that access.

For sample apps, you make a good point indeed about the existing app not being
very useful as they work on larger packets. I'll see what I can throw together
here to make a more realistic test.

/Bruce
 
> 
> 04.09.2014 16:27, Bruce Richardson ??????????:
> > Hi Yerden,
> >
> > I understand your concerns and it's good to have this discussion.
> >
> > There are a number of reasons why I've moved these particular fields
> > to the second cache line. Firstly, the main reason is that, obviously enough,
> > not all fields will fit in cache line 0, and we need to prioritize what does
> > get stored there. The guiding principle behind what fields get moved or not
> > that I've chosen to use for this patch set is to move fields that are not
> > used on the receive path (or the fastpath receive path, more specifically -
> > so that we can move fields only used by jumbo frames that span mbufs) to the
> > second cache line. From a search through the existing codebase, there are no
> > drivers which set the l2/l3 length fields on RX, this is only used in
> > reassembly libraries/apps and by the drivers on TX.
> >
> > The other reason for moving it to the second cache line is that it logically
> > belongs with all the other length fields that we need to add to enable
> > tunneling support. [To get an idea of the extra fields that I propose adding
> > to the mbuf, please see the RFC patchset I sent out previously as "[RFC 
> > PATCH 00/14] Extend the mbuf structure"]. While we probably can fit the 16-bits
> > needed for l2/l3 length on the mbuf line 0, there is not enough room for all
> > the lengths so we would end up splitting them with other fields in between.
> >
> > So, in terms of what do to about this particular issue. I would hope that for
> > applications that use these fields the impact should be small and/or possible
> > to work around e.g. maybe prefetch second cache line on RX in driver. If not,
> > then I'm happy to see about withdrawing this particular change and seeing if
> > we can keep l2/l3 lengths on cache line zero, with other length fields being
> > on cache line 1.
> >
> > Question: would you consider the ip fragmentation and reassembly example apps
> > in the Intel DPDK releases good examples to test to see the impacts of this
> > change, or is there some other test you would prefer that I look to do? 
> > Can you perhaps test out the patch sets for the mbuf that I've upstreamed so
> > far and let me know what regressions, if any, you see in your use-case
> > scenarios?
> >
> > Regards,
> > /Bruce
> >
> -- 
> Sincerely,
> 
> Yerden Zhumabekov
> STS, ACI
> Astana, KZ
>