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From Neil Conway <neil.con...@gmail.com>
Subject Re: Review Request 43635: Changed scalar resources to use fixed-point internally.
Date Fri, 26 Feb 2016 23:06:01 GMT


> On Feb. 17, 2016, 12:30 a.m., Klaus Ma wrote:
> > src/common/values.cpp, line 67
> > <https://reviews.apache.org/r/43635/diff/1/?file=1252182#file1252182line67>
> >
> >     Let's add check on overflow; it will be helpful if scalar value was big. Scalar
is a general type, there maybe used with a big value, e.g. total size of distributed filesystem.
> 
> Neil Conway wrote:
>     What should we do in case of overflow?
>     
>     Note that we don't check for overflow in operator+ (or for negative result values
in operator-)...
> 
> Klaus Ma wrote:
>     `CHECK` or warning log should be fine, it only improves the debugability; it different
with `operator+` because `max_double/2` should be big enough for us; but for `double * 1000
-> long`, I'm not sure of that.
>     
>     And I think we need to use `long long` or `int64` instead of `long`; `long` is not
garantee to be 64bit in c++.
> 
> Neil Conway wrote:
>     re: `long`, good point -- we only officially support 64-bit OSX, Linux, and Windows,
but `long` is 32-bit on Windows.

After discussing this with a few folks offline, our feeling was that the range of resource
values we can represent without overflow should be sufficient for any reasonable purpose for
a considerable length of time. Hopefully we'll have switched to a fixed point format before
people use Mesos to manage > 10s of exabytes as a single resource :)


- Neil


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On Feb. 19, 2016, 10:27 p.m., Neil Conway wrote:
> 
> -----------------------------------------------------------
> This is an automatically generated e-mail. To reply, visit:
> https://reviews.apache.org/r/43635/
> -----------------------------------------------------------
> 
> (Updated Feb. 19, 2016, 10:27 p.m.)
> 
> 
> Review request for mesos, Joris Van Remoortere and Michael Park.
> 
> 
> Bugs: MESOS-4687
>     https://issues.apache.org/jira/browse/MESOS-4687
> 
> 
> Repository: mesos
> 
> 
> Description
> -------
> 
> 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
> -----
> 
>   docs/attributes-resources.md 818da8ab0c672144b02f526b2b805cf0505d2c7e 
>   docs/upgrades.md 4f30d725c6ed28c09a1c5528fd4193c3f06b2d93 
>   include/mesos/mesos.proto 636550f255a122d7f714dbd58f587bea8221b461 
>   include/mesos/v1/mesos.proto 1d5af88a343fe9d81688bb3e83aa997ccba7d030 
>   src/common/resources.cpp 5d731870542166cec11f9956ccdf16207b2d22cc 
>   src/common/values.cpp c64407bc97ad858300f4661d616e0480920fc541 
>   src/master/allocator/mesos/hierarchical.cpp 5ef29f26ec8071f79c2f4f78dbe2bb0a613cc92d

>   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
> -------
> 
> 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.
> 
> PERFORMANCE:
> 
> This is the performance of the `DeclineOffers` benchmark WITHOUT this RR applied (optimized
build on my laptop):
> 
> ```
> [ RUN      ] HierarchicalAllocator_BENCHMARK_Test.DeclineOffers
> Using 2000 slaves and 200 frameworks
> round 0 allocate took 2.192425secs to make 200 offers
> round 1 allocate took 2.221243secs to make 200 offers
> round 2 allocate took 2.236314secs to make 200 offers
> round 3 allocate took 2.224045secs to make 200 offers
> round 4 allocate took 2.232822secs to make 200 offers
> round 5 allocate took 2.264807secs to make 200 offers
> round 6 allocate took 2.224853secs to make 200 offers
> round 7 allocate took 2.224829secs to make 200 offers
> round 8 allocate took 2.24862secs to make 200 offers
> round 9 allocate took 2.2556secs to make 200 offers
> round 10 allocate took 2.256616secs to make 200 offers
> ```
> 
> And after applying this RR:
> 
> ```
> [ RUN      ] HierarchicalAllocator_BENCHMARK_Test.DeclineOffers
> Using 2000 slaves and 200 frameworks
> round 0 allocate took 2.267919secs to make 200 offers
> round 1 allocate took 2.202843secs to make 200 offers
> round 2 allocate took 2.20426secs to make 200 offers
> round 3 allocate took 2.263887secs to make 200 offers
> round 4 allocate took 2.266237secs to make 200 offers
> round 5 allocate took 2.276957secs to make 200 offers
> round 6 allocate took 2.291821secs to make 200 offers
> round 7 allocate took 2.261839secs to make 200 offers
> round 8 allocate took 2.325696secs to make 200 offers
> round 9 allocate took 2.310469secs to make 200 offers
> round 10 allocate took 2.21802secs to make 200 offers
> ```
> 
> Which suggests to me that any performance hit is pretty minimal.
> 
> 
> Thanks,
> 
> Neil Conway
> 
>


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