On Sun, Jan 28, 2018 at 05:43:34PM +0100, Sjors Provoost via bitcoin-dev wrote: > Peter Todd wrote: > > In fact I considered only requiring an increase in fee rate, based on the > > theory that if absolute fee went down, the transaction must be smaller and > > thus > > miners could overall earn more from the additional transactions they could > > fit > > into their block. But to do that properly requires considering whether or > > not > > that's actually true in the particular state the mempool as a whole happens > > to > > be in, so I ditched that idea early on for the much simpler criteria of > > both a > > feerate and absolute fee increase. > > Why would you need to consider the whole mempool?
Imagine a miner is only concerned with creating the next block and his mempool currently only has 750,000 vbytes in it. If two 250-vbyte transactions each paying a feerate of 100 nanobitcoins per vbyte (50k total) are replaced with one 325-vbyte transaction paying a feerate of 120 nBTC (39k total), the miner's potential income from mining the next block is reduced by 11k nBTC. Moving away from this easily worked example, the problem can still exist even if a miner has enough transactions to fill the next block. For replacement consideration only by increased feerate to be guaranteed more profitable, one has to assume the mempool contains an effectively continuous distribution of feerates. That may one day be true of the mempool (it would be good, because it helps keep block production regular sans subsidy) but it's often not the case these days. -Dave
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