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