Re: [dpdk-dev] 答复: [PATCH] Add user defined tag calculation callback tolibrte_distributor.

[jigsaw <jigsaw@gmail.com>]

Hi Bruce,

In my use case, unfortunately the tag is not hash. And the tag can be on
either low or high bits, depending on configuration.
I wonder if it is possible to let the user to decide which bit to mask,
i.e. to add another param to rte_distributor_create to define the mask.

thx &
rgds,
-qinglai

On Thu, Nov 6, 2014 at 3:59 PM, Bruce Richardson <bruce.richardson@intel.com
> wrote:

> On Thu, Nov 06, 2014 at 02:36:09PM +0200, Qinglai Xiao wrote:
> > Hi Bruce,
> >
> > There is a subtle case in which tag values are 2 and 3, respectively.
> Then these two tags cannot be distinguished. There should be a better way
> so as to handle this situation.
>
> It's not just in that, case, it's in any case where a pair of tags differ
> by
> only a single bit. I've been assuming that the tag is likely to be a hash
> value in most cases - given that it's only 32-bit - in which case it just
> doesn't
> matter which bit we chose to permanently set to 1, but if there are
> scenarios
> where it's likely that the low bits are used but the high ones not so, we
> can
> look to change which bit is set to 1. Either way, the distributor just
> uses a
> 31-bit tag rather than a 32-bit one.
>
> /Bruce
>
> >
> > thx &
> > rgds
> > -qinglai
> >
> > -----原始邮件-----
> > 发件人: "Thomas Monjalon" <thomas.monjalon@6wind.com>
> > 发送时间: ‎2014/‎11/‎6 12:36
> > 收件人: "Bruce Richardson" <bruce.richardson@intel.com>
> > 抄送: "dev@dpdk.org" <dev@dpdk.org>; "jigsaw" <jigsaw@gmail.com>
> > 主题: Re: [dpdk-dev] [PATCH] Add user defined tag calculation callback
> tolibrte_distributor.
> >
> > 2014-11-06 09:22, Bruce Richardson:
> > > On Wed, Nov 05, 2014 at 07:24:13PM +0200, jigsaw wrote:
> > > >
> http://dpdk.org/browse/dpdk/tree/lib/librte_distributor/rte_distributor.c#n285
> > > >
> > > >         new_tag = (next_mb->hash.rss | 1);
> > > >
> > > > Why the logical OR is needed?
> > >
> > > That's needed to ensure that we never track a tag with an actual value
> of zero.
> > > We instead always force the low bit to be 1, so that we can use zero
> as an
> > > "empty" value.
> >
> > Bruce, could you check how this code may be better commented please?
> > This discussion shows that the distributor library probably needs more
> > explanations in the code or doxygen.
> >
> > Thanks
> > --
> > Thomas
>