From: Jonathan Toomim <j@toom.im>
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Date: Tue, 27 Oct 2015 21:26:52 -0700
Message-Id: <291B85A6-D8D4-443B-B03B-C675CBEEC662@toom.im>
To: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
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Subject: [bitcoin-dev] Composite priority: combining fees and bitcoin-days
	into one number
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Assigning 5% of block space based on bitcoin-days destroyed (BDD) and =
the other 95% based on fees seems like a rather awkward approach to me. =
For one thing, it means two code paths in pretty much every procedure =
dealing with a constrained resource (e.g. mempool, CNB). This makes code =
harder two write, harder to maintain, and slower to execute. As a =
result, some people have proposed eliminating BDD priority altogether. I =
have another idea.

We can create and maintain a conversion rate between BDD and fees to =
create a composite priority metric. Then we just do compPrio =3D BDD * =
conversionRate + txFee.

How do we calculate conversionRate? We want the following equation to be =
true:

sum(fees) =3D sum(BDD) * conversionRate * BDDweight

So we sum up the mempool fees, and we sum up the mempool BDD. We get a =
policy statement from the command line for a relative weight of BDD vs =
fees (default 0.05), and then conversionRate =3D (summedFees / =
summedBDD) * BDDWeight.

As an optimization, rather than scanning over the whole mempool to =
calculate this, we can just store the sum and add or subtract from it =
each time a tx enters or leaves the mempool. In order to minimize drift =
(the BDD for a transaction changes over time), we recalculate the whole =
thing each time a new block is found.

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