Re: [dpdk-dev] [PATCH 0/3] add new Double Word Key hash table

From: "Morten Brørup" <mb@smartsharesystems.com>
To: "Medvedkin, Vladimir" <vladimir.medvedkin@intel.com>, <dev@dpdk.org>
Cc: <konstantin.ananyev@intel.com>, <yipeng1.wang@intel.com>,
	<sameh.gobriel@intel.com>, <bruce.richardson@intel.com>,
	"Suanming Mou" <suanmingm@mellanox.com>,
	"Olivier Matz" <olivier.matz@6wind.com>,
	"Xueming(Steven) Li" <xuemingl@mellanox.com>,
	"Andrew Rybchenko" <arybchenko@solarflare.com>,
	"Asaf Penso" <asafp@mellanox.com>, "Ori Kam" <orika@mellanox.com>
Subject: Re: [dpdk-dev] [PATCH 0/3] add new Double Word Key hash table
Date: Mon, 16 Mar 2020 20:33:23 +0100	[thread overview]
Message-ID: <98CBD80474FA8B44BF855DF32C47DC35C60EDC@smartserver.smartshare.dk> (raw)
In-Reply-To: <2baf9a76-a606-d376-dcce-b95c80fc6bb2@intel.com>

> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Medvedkin, Vladimir
> Sent: Monday, March 16, 2020 7:28 PM
> 
> Hi Morten,
> 
> 
> On 16/03/2020 14:39, Morten Brørup wrote:
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Vladimir Medvedkin
> >> Sent: Monday, March 16, 2020 2:38 PM
> >>
> >> Currently DPDK has a special implementation of a hash table for
> >> 4 byte keys which is called FBK hash. Unfortunately its main drawback
> >> is that it only supports 2 byte values.
> >> The new implementation called DWK (double word key) hash
> >> supports 8 byte values, which is enough to store a pointer.
> >>
> >> It would also be nice to get feedback on whether to leave the old FBK
> >> and new DWK implementations, or whether to deprecate the old one?
> > <rant on>
> >
> > Who comes up with these names?!?
> >
> > FBK (Four Byte Key) and DWK (Double Word Key) is supposed to mean the
> same. Could you use 32 somewhere in the name instead, like in int32_t,
> instead of using a growing list of creative synonyms for the same thing?
> Pretty please, with a cherry on top!
> 
> 
> That's true, at first I named it as fbk2, but then it was decided to
> rename it "dwk", so that there was no confusion with the existing FBK
> library. Naming suggestions are welcome!
> 

OK... let me suggest a prefix with both key and value sizes in the name:
Instead of rte_dwk_hash_xxx, use rte_hash_k32v64_xxx.

However, that suggests that it is a generic hash... and your hash algorithm certainly has other properties too; so perhaps a specific name, saying something about the underlying algorithm, makes more sense after all. And the documentation will show its properties anyway.

Whatever you choose, Double Word is certainly not a good name.... a word is 32 bits and a double word is 64 bits in many non-Intel CPU architectures, including some supported by DPDK.

> >
> > And if the value size is fixed too, perhaps the name should also indicate
> the value size.
> >
> > <rant off>
> >
> > It's a shame we don't have C++ class templates available in DPDK...
> >
> > In other news, Mellanox has sent an RFC for an "indexed memory pool"
> library [1] to conserve memory by using uintXX_t instead of pointers, so
> perhaps a variant of a 32 bit key hash library with 32 bit values (in
> addition to 16 bit values in FBK and 64 bit in DWK) would be nice
> combination with that library.
> >
> > [1]: http://mails.dpdk.org/archives/dev/2019-October/147513.html
> >
> >
> > Med venlig hilsen / kind regards
> > - Morten Brørup
> >
> --
> Regards,
> Vladimir
> 