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

["Medvedkin, Vladimir" <vladimir.medvedkin@intel.com>]

Hi Honnappa,

-----Original Message-----
From: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com> 
Sent: Tuesday, March 31, 2020 10:17 PM
To: thomas@monjalon.net; Medvedkin, Vladimir <vladimir.medvedkin@intel.com>
Cc: Wang, Yipeng1 <yipeng1.wang@intel.com>; Stephen Hemminger <stephen@networkplumber.org>; dev@dpdk.org; Morten Brørup <mb@smartsharesystems.com>; dev@dpdk.org; Ananyev, Konstantin <konstantin.ananyev@intel.com>; Gobriel, Sameh <sameh.gobriel@intel.com>; Richardson, Bruce <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>; nd <nd@arm.com>; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; nd <nd@arm.com>
Subject: RE: [dpdk-dev] [PATCH 0/3] add new Double Word Key hash table

<snip>

> 
> 26/03/2020 18:28, Medvedkin, Vladimir:
> > Hi Yipeng, Stephen, all,
> >
> > On 17/03/2020 19:52, Wang, Yipeng1 wrote:
> > > From: Stephen Hemminger <stephen@networkplumber.org>
> > >> On Mon, 16 Mar 2020 18:27:40 +0000 "Medvedkin, Vladimir" 
> > >> <vladimir.medvedkin@intel.com> wrote:
> > >>
> > >>> 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!
> > >>>
> > >>>> 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
> 
> Yes some work is in progress to propose a new memory allocator for 
> small objects of fixed size with small memory overhead.
> 
> 
> > >> Why is this different (or better) than existing rte_hash.
> > >> Having more flavors is not necessarily a good thing (except in
> > >> Gelato)
> > >   [Wang, Yipeng]
> > > Hi, Vladimir,
> > > As Stephen mentioned, I think it is good idea to explain the 
> > > benefit of this new type of hash table more explicitly such as 
> > > Specific use cases,
> differences with current rte_hash, and performance numbers, etc.
> >
> > The main reason for this new hash library is performance. As I 
> > mentioned earlier, the current rte_fbk implementation is pretty fast 
> > but it has a number of drawbacks such as 2 byte values and limited 
> > collision resolving capabilities. On the other hand, rte_hash 
> > (cuckoo
> > hash) doesn't have this drawbacks but at the cost of lower 
> > performance comparing to rte_fbk.
> >
> > If I understand correctly, performance penalty are due to :
> >
> > 1. Load two buckets
> >
> > 2. First compare signatures
> >
> > 3. If signature comparison hits get a key index and find memory 
> > location with a key itself and get the key
> >
> > 4. Using indirect call to memcmp() to compare two uint32_t.
> >
> > The new proposed 4 byte key hash table doesn't have rte_fbk 
> > drawbacks while offers the same performance as rte_fbk.
> >
> > Regarding use cases, in rte_ipsec_sad we are using rte_hash with 4 
> > byte key size. Replacing it with a new implementation gives about 
> > 30% in performance.
> >
> > The main disadvantage comparing to rte_hash is some performance 
> > degradation with high average table utilization due to chain 
> > resolving for 5th and subsequent collision.
rte_hash is linearly scalable across multiple cores for lookup due to lock-free algorithm. How is the scalability for the new algorithm?

This library is scalable as well. It uses almost the same lock-free algorithm. The only difference with cuckoo is that cuckoo in lock-free implementation uses single global "change_counter" for all the table, and the proposed implementation uses fine grained approach with "change_counter" per bucket. So it should be more scalable with frequent concurrent updates.

> 
> Thanks for explaining.
> Please, such information should added in the documentation:
> 	doc/guides/prog_guide/hash_lib.rst
> 
>