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

[jigsaw <jigsaw@gmail.com>]

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

On Thu, Nov 6, 2014 at 8:01 PM, jigsaw <jigsaw@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@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
>>
>
>