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From Neil Conway <neil.con...@gmail.com>
Subject Review Request 43635: Changed scalar resources to use fixed-point internally.
Date Tue, 16 Feb 2016 23:27:54 GMT

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This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/43635/
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Review request for mesos and Joris Van Remoortere.


Bugs: MESOS-4687
    https://issues.apache.org/jira/browse/MESOS-4687


Repository: mesos


Description
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Scalar resource values are represented using floating point. As a result, users
could see unexpected results when accepting offers and making reservations for
fractional resources: values like "0.1" cannot be precisely represented using
standard floating point, and the resource values returned to frameworks might
contain an unpredictable amount of roundoff error.

This commit adjusts the master to use fixed-point when doing internal
computations on scalar resource values. The fixed-point format only supports
three decimal digits of precision: that is, fractional resource values like
"0.001" will be supported, but "0.0001" will not be.


Diffs
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  docs/attributes-resources.md 818da8ab0c672144b02f526b2b805cf0505d2c7e 
  docs/upgrades.md 4f30d725c6ed28c09a1c5528fd4193c3f06b2d93 
  include/mesos/mesos.proto e24d3e03a7dc7c6bfd07f34531cb593fe4925646 
  include/mesos/v1/mesos.proto d909e60ddfd8e3ba2075f82c372edde04cd99d54 
  src/common/resources.cpp 5d731870542166cec11f9956ccdf16207b2d22cc 
  src/common/values.cpp c64407bc97ad858300f4661d616e0480920fc541 
  src/master/allocator/mesos/hierarchical.cpp a9d2c23162892e22220f97d89a076d2311091d91 
  src/tests/resources_tests.cpp 96864c3945729fe33be8b243b9c826fb12e90eff 
  src/v1/resources.cpp 207eb61d6a6d03d314539d42751cac65fcffa9af 
  src/v1/values.cpp 58ea9875804bf0287855a1e9855855e5e54de4c4 

Diff: https://reviews.apache.org/r/43635/diff/


Testing
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make check

Manually verified that some of the floating point oddities in https://issues.apache.org/jira/browse/MESOS-4071
do not occur when this patch is applied, although I wasn't able to reproduce the crash described
in that ticket.

REVIEW NOTES:

* We don't currently emit a warning when discarding additional digits of precision from input
scalar resource values. Should we? That would require identifying all the points where a resource
value is seemed to be "user-provided", and also runs the risk of generating a ton of log messages
when an old framework is used.
* Similarly, if the user gives us a resource value and we don't do anything to it, we won't
discard any additional precision that appears in the value -- the precision only gets discarded
when we apply an operator like `+` or `-`. Unclear if we should trim additional precision
from all scalar resource values more aggressively.


Thanks,

Neil Conway


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