Re: [PATCH 0/2] rewritten rte_hash_crc() call

[Yerden Zhumabekov <e_zhumabekov-8EHiFRVJVgQ@public.gmane.org>]

14.11.2014 17:33, Neil Horman пишет:
> On Fri, Nov 14, 2014 at 01:15:12PM +0600, Yerden Zhumabekov wrote:
>>
>> Hello,
>>
>> A quick grep on dpdk source shows that rte_hash_crc() is used in
>> librte_hash in following context:
>>
>> In rte_hash.c:
>> /* Hash function used if none is specified */
>> #ifdef RTE_MACHINE_CPUFLAG_SSE4_2
>> #include <rte_hash_crc.h>
>> #define DEFAULT_HASH_FUNC       rte_hash_crc
>> #else
>> #include <rte_jhash.h>
>> #define DEFAULT_HASH_FUNC       rte_jhash
>> #endif
>>
>> In rte_fbk_hash.h
>> #ifdef RTE_MACHINE_CPUFLAG_SSE4_2
>> #include <rte_hash_crc.h>
>> /** Default four-byte key hash function if none is specified. */
>> #define RTE_FBK_HASH_FUNC_DEFAULT·······rte_hash_crc_4byte
>> #else
>> #include <rte_jhash.h>
>> #define RTE_FBK_HASH_FUNC_DEFAULT·······rte_jhash_1word
>> #endif
>> #endif
>>
>>
>> I guess it covers the cpu flags check you're talking about.
>>
> Not really.  That covers the case of applications selecting the hash function
> using the DEFUALT_HASH_FUNC macro, but doesn't nothing for applications using
> the function directly.  Test_hash_perf is an example  of this, and ostensibly
> because of the behavior without SSE4.2 it defines these huge test tables twice
> based on the availability of SSE4.2.  It would be better if we could allow
> applications to use rte_hash_crc regardless, and make the code it uses at run
> time configurable.

I see, then we have a problem here :)

Actually, that was one of my concerns when developing these patches. I
looked through the source code of libs and examples and I saw the
'#ifdef..#include..#endif'-like appoach while selecting hash function
was common. So I organized patches to minimize the impact on API and not
to contradict this approach.

If we prefer to change this approach then, I guess, we need to introduce
broader changes to rte_hash library and change other code which uses it.
If that's what's needed, then it'll take some time for me to rework
these patches.

-- 
Sincerely,

Yerden Zhumabekov
State Technical Service
Astana, KZ