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

[Jianbo Liu <jianbo.liu@linaro.org>]

On 4 July 2017 at 21:55, De Lara Guarch, Pablo
<pablo.de.lara.guarch@intel.com> wrote:
>
>
>> -----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).
>

I think Cristian explanation justified using the special hash functions here.
And they may have better performance than the standard functions in the library.

Thanks!
Jianbo