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 at intel.com> 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 at gmail.com> 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 at 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 > > > >> > > > > >> > -----????----- > > > >> > ???: "Thomas Monjalon" <thomas.monjalon at 6wind.com> > > > >> > ????: ?2014/?11/?6 12:36 > > > >> > ???: "Bruce Richardson" <bruce.richardson at intel.com> > > > >> > ??: "dev at dpdk.org" <dev at dpdk.org>; "jigsaw" <jigsaw at > > > >> > gmail.com> > > > >> > ??: 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 > > > >> > > > > > > > > > >
