Re: [dpdk-dev] [PATCH 2/2] mbuf: make sure userdata is initialized

[Olivier MATZ <olivier.matz@6wind.com>]

On 07/10/2015 05:43 PM, Stephen Hemminger wrote:
> On Fri, 10 Jul 2015 10:32:17 +0100
> Bruce Richardson <bruce.richardson@intel.com> wrote:
>
>> On Thu, Jul 09, 2015 at 04:37:48PM -0700, Stephen Hemminger wrote:
>>> From: Stephen Hemminger <shemming@brocade.com>
>>>
>>> For applications that use m->userdata the initialization can
>>> be a signficant (10%) performance penalty.
>>>
>>> Rather than taking the cache penalty of initializing userdata
>>> in the receive handling, do it in the place where mbuf is
>>> already cache hot and being setup.
>>
>> Should the management of the userdata field not be the responsibility of the
>> app itself, rather than having the PMD manage it? If the PMD does manage the
>> userdata field, I would suggest taking the approach of having the field cleared
>> by the mbuf library on free, rather than on RX.
>
> The problem with that is m->userdata is that when the application
> gets the mbuf, touching the userdata field causes false cache
> sharing and we see a significant performance drop. Internally the
> userdata field is only used for few special cases and 0/NULL
> is used to indicate that no metadata is present.
>

I agree with Bruce: the userdata field is in the second cache line,
and we should avoid touching it in rx path. Resetting it in mbuf_free()
may not be the solution either: in tx path, the mbufs are freed when
we want to send a new mbuf on the hw ring entry, and the mbuf may
not be in the cache anymore.

I don't understand why doing it in the application induces a performance
drop compared to the PMD. What do you mean by false cache sharing?