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From "Taras Bobrovytsky (Code Review)" <ger...@cloudera.org>
Subject [Impala-ASF-CR] IMPALA-4939, IMPALA-4939: Decimal V2 multiplication
Date Thu, 03 Aug 2017 20:13:47 GMT
Taras Bobrovytsky has posted comments on this change.

Change subject: IMPALA-4939, IMPALA-4939: Decimal V2 multiplication
......................................................................


Patch Set 4:

(1 comment)

http://gerrit.cloudera.org:8080/#/c/7438/4//COMMIT_MSG
Commit Message:

Line 75: DECIMAL_V2 disabled: 2.65s
> This is scary and I don't think we should consider merging this until it's 
It takes time to convert a number from 128 bits to 256 bits. Also, Boost does something like
4 128 bit multiplications internally in the 256 bit multiplication function (so it makes sense
for it to be at least 4x slower). I think the 256 bit multiplication function is also not
optimized for the worst case (where the result actually needs more than 128 bits).

Also, it takes time to convert 2 numbers to 256 bit numbers and then convert 1 number back.

I don't think it's that surprising that it's much slower. 256 bit multiplication is slow and
is not implemented in hardware like others.

Remember, this benchmark is the absolute worst case scenario, where we do a 256 bit multiplication
for every row.


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Gerrit-MessageType: comment
Gerrit-Change-Id: I37ad6232d7953bd75c18dc86e665b2b501a1ebe1
Gerrit-PatchSet: 4
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Gerrit-Reviewer: Dan Hecht <dhecht@cloudera.com>
Gerrit-Reviewer: Michael Ho <kwho@cloudera.com>
Gerrit-Reviewer: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Gerrit-Reviewer: Tim Armstrong <tarmstrong@cloudera.com>
Gerrit-Reviewer: Zach Amsden <zamsden@cloudera.com>
Gerrit-HasComments: Yes

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