Miner Policy: Correct truncating division

[wadealexc]

QualityForWeight returns the average of weighted space time here:

specs-actors/actors/builtin/miner/policy.go

Lines 184 to 185 in 6c71cf5

	// Average of weighted space time: (scaledUpWeightedSumSpaceTime / sectorSpaceTime * 10)
	return big.Div(big.Div(scaledUpWeightedSumSpaceTime, sectorSpaceTime), builtin.QualityBaseMultiplier)

Dividing twice may introduce error in some cases. The return can be re-written to divide only once:

return big.Div(scaledUpWeightedSumSpaceTime, big.Mul(sectorSpaceTime, builtin.QualityBaseMultiplier))

[anorth]

IMO this is worth fixing up in the next major version update. This function is called quite frequently, so avoiding the extra division may make a just-noticeable performance improvement too. But the frequent use makes plumbing the network version through to do it in a minor upgrade rather painful.

[ZenGround0]

This needs analysis or a network upgrade to be safe in case power values change between the two calculations.

Approach 0: we could mathematically guarantee that the loss in precision is never enough to cause a difference between power values of sectors. Then this is purely an optimization trading a Div for a Mul. It's probably the case we can bound error (SectorQualityPrecision gives us 20 bits of room) and that Div's are more expensive than Muls so we should start here.

Approach 1: Write a migration traversing all partitions / deadlines and rerunning all power calculations over all sectors to recompute power value. This traverses essentially all state blocks (sector infos + deadline trees)

Approach2: Migration only recomputes partition power values if partition power is not the same as (constant power of cc sector) * (number of sectors) because this is the only case the power calculations will be different. This relies on the fact that QAPowerForWeight will always divide out exact multiples when there is no verified deal weight.