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

[Bruce Richardson <bruce.richardson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>]

On Fri, Nov 07, 2014 at 02:38:13PM +0200, jigsaw wrote:
> Hi Bruce,
> 
> >>  If a tag value of zero is ever passed in, then it will start matching
> against cores which are not doing any processing.
> 
> Yes, this is true according to current bookkeeping of inflight tags.
> 
> But if the slot in in_flight_tags is not a uint32_t but a struct which has
> a filed as indication of "on/off", and also with corresponding changes in
> looking for a matched tag, then the need for 1 bit mask can be eliminated.
> Of course this change requires a little bit more, O(n), memory space and
> costs O(n) more branch misses. But the benefit is a more free interface to
> user app.
> 
> This is just another trade-off. Since I am in need of such freedom, I am
> more interested in the free use of 32bits.

If you do implement such a change, I would suggest you simply add a bitmask
to the distributor indicating valid workers. Then when we do the check
for tag matches, we just need an extra "and" instruction to eliminate invalid
workers from the match.

/Bruce

> 
> thx &
> rgds,
> -qinglai
> 
> 
> On Fri, Nov 7, 2014 at 11:45 AM, Bruce Richardson <
> bruce.richardson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> wrote:
> 
> > On Thu, Nov 06, 2014 at 09:52:25PM +0200, jigsaw wrote:
> > > Hi Bruce,
> > >
> > > Actually IMHO it is good to leave the freedom to user to decide how to
> > > interpret the tag value, i.e. remove the OR 1 bit.
> > > If the tag value is zero, then we assume the programmer know what he is
> > > doing. Of course this shall be clearly documented in the comment/doxgen.
> > >
> > >
> > > thx &
> > > rgds,
> > > -qinglai
> >
> > I don't believe that will work. If a tag value of zero is ever passed
> > in, then it will start matching against cores which are not doing any
> > processing. Then it will get queued up to get sent to those cores, and so
> > never get processed.
> > We need a bit somewhere inside the tag to permanently set - though it can
> > be configurable.
> >
> > /Bruce
> >
> > >
> > > On Thu, Nov 6, 2014 at 8:01 PM, jigsaw <jigsaw-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > >
> > > > 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-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> 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-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> > > >> > 抄送: "dev-VfR2kkLFssw@public.gmane.org" <dev-VfR2kkLFssw@public.gmane.org>; "jigsaw" <jigsaw-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
> > > >> > 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
> > > >>
> > > >
> > > >
> >