Re: [dpdk-dev] [PATCH] examples/ip_pipeline: use crc32 in hash functions for arm64

["De Lara Guarch, Pablo" <pablo.de.lara.guarch@intel.com>]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas@monjalon.net]
> Sent: Tuesday, July 4, 2017 12:26 AM
> To: Dumitrescu, Cristian <cristian.dumitrescu@intel.com>; De Lara Guarch,
> Pablo <pablo.de.lara.guarch@intel.com>
> Cc: dev@dpdk.org; Jianbo Liu <jianbo.liu@linaro.org>;
> jerin.jacob@caviumnetworks.com; ashwin.sekhar@caviumnetworks.com
> Subject: Re: [dpdk-dev] [PATCH] examples/ip_pipeline: use crc32 in hash
> functions for arm64
> 
> 04/07/2017 01:19, Dumitrescu, Cristian:
> > From: Thomas Monjalon [mailto:thomas@monjalon.net]
> > > 18/05/2017 11:09, Jianbo Liu:
> > > > Implement the same hash functions with crc32 on arm platform.
> > > >
> > > > Signed-off-by: Jianbo Liu <jianbo.liu@linaro.org>
> > > > ---
> > > >  examples/ip_pipeline/pipeline/hash_func.h       |   2 +
> > > >  examples/ip_pipeline/pipeline/hash_func_arm64.h | 245
> > > ++++++++++++++++++++++++
> > > >  2 files changed, 247 insertions(+)  create mode 100644
> > > > examples/ip_pipeline/pipeline/hash_func_arm64.h
> > >
> > > I don't understand why this code is in an example.
> > > We have some CRC code in librte_hash, librte_net and ip_pipeline.
> > > Cristian, Jianbo,
> > > does it make sense to move these functions somewhere else?
> > >
> >
> > I think example apps are a great way to propose new hash functions.
> > IMO we should encourage the definition/exploration of new hash
> functions in our example apps.
> >
> > These functions are examples of how fast hash functions can be built
> using special CPU instructions.
> > They have much better performance than e.g. jhash, but their
> > properties are largely unknown, as no rigorous study on their
> > properties (such as uniform distribution) has been conducted. I have
> > seen them providing good performance  for the data set I have been
> using, but I have no extensive data to support their maturity level.
> >
> > If somebody is willing to invest the effort in proving them, I would
> > be more than happy to see them moved to a library like librte_hash.
> > Pablo as maintainer has the choice (I think it is not the first time
> > we discuss bout these hash funcs :) )
> >
> > As mentioned in one of our deprecation notices, I am actively working
> > (not ready for 17.8 unfortunately) to add a key mask parameter to these
> functions, so more work on these hash functions is likely to take place.
> 
> OK thanks for the explanation.
> I still think we do not need to prove hash for integrating them.
> I would be interested to read Pablo's opinion.

If these functions are used as hash functions, I would place them in rte_hash.

The case where we placed the CRC function in librte_net was because that
was not used as a hash function, so it made sense to me placing it there,
but in this case, it looks like it is, so I think rte_hash is a valid place
(although someone would need to integrate it with the existing CRC hash function in that library).

Thanks,
Pablo