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Parallel sum produces different result from sequential sum #4958

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scabug opened this issue Sep 1, 2011 · 4 comments
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Parallel sum produces different result from sequential sum #4958

scabug opened this issue Sep 1, 2011 · 4 comments
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scabug commented Sep 1, 2011

Summing list sequentially and in parallel produces two different results.

The following test fails:

// summands of Leibniz formula for pi
val list = 0 until 10000 map { i => BigDecimal(4) * (1 - (i % 2) * 2) / (2 * i + 1) }

val piSeq = list sum
val piPar = list.par sum

assert(piSeq == piPar)
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scabug commented Sep 1, 2011

Imported From: https://issues.scala-lang.org/browse/SI-4958?orig=1
Reporter: Sanjin Sehic (saserr)
Affected Versions: 2.9.1
See #4658

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scabug commented Sep 1, 2011

Sanjin Sehic (saserr) said:
Additionally, replacing {{sum}} with {{reduceLeft {_ + _}}} for parallel computation returns same result as sequential one.

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scabug commented Sep 1, 2011

@soc said:
I guess even BigDecimals are not commutative under addition.

This is related to #4658, because for your actual example neither sum nor reduceLeft are necessary, because the result can be determined algorithmically.

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scabug commented Sep 4, 2011

@paulp said:
Not a bug. Create your BigDecimals with java.math.MathContext.UNLIMITED and you will get the same result from both (both will throw exceptions for non-terminating decimal expansions.) When you perform rounding operations in different orders, you get different results.

@scabug scabug closed this as completed Sep 4, 2011
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