James,

You seem to imply that the scenario described isn't prevented today. It is. The mempool acceptance for a replacement not only
depend on the transaction feerate but also the transaction fee [0]. That's why i raised it in the first place...

Antoine

[0] https://github.com/bitcoin/bitcoin/blob/66636ca438cb65fb18bcaa4540856cef0cee2029/src/validation.cpp#L944-L947

Of course if you are evicting transactions then you don't have the issue i mentioned, so it's fine doing so.
-------- Original Message --------
On Feb 17, 2022, 19:18, James O'Beirne < james.obeirne@gmail.com> wrote:

>> Is it really true that miners do/should care about that?
>
> De facto, any miner running an unmodified version of bitcoind doesn't
> care about anything aside from ancestor fee rate, given that the
> BlockAssembler as-written orders transactions for inclusion by
> descending ancestor fee-rate and then greedily adds them to the block
> template. [0]
>
> If anyone has any indication that there are miners running forks of
> bitcoind that change this behavior, I'd be curious to know it.
>
> Along the lines of what AJ wrote, optimal transaction selection is
> NP-hard (knapsack problem). Any time that a miner spends deciding how
> to assemble the next block is time not spent grinding on the nonce, and
> so I'm skeptical that miners in practice are currently doing anything
> that isn't fast and simple like the default implementation: sorting
> fee-rate in descending order and then greedily packing.
>
> But it would be interesting to hear evidence to the contrary.
>
> ---
>
> You can make the argument that transaction selection is just a function
> of mempool contents, and so mempool maintenance criteria might be the
> thing to look at. Mempool acceptance is gated based on a minimum
> feerate[1]. Mempool eviction (when running low on space) happens on
> the basis of max(self_feerate, descendant_feerate) [2]. So even in the
> mempool we're still talking in terms of fee rates, not absolute fees.
>
> That presents us with the "is/ought" problem: just because the mempool
> *is* currently gating only on fee rate doesn't mean that's optimal. But
> if the whole point of the mempool is to hold transactions that will be
> mined, and if there's good reason that txns are chosen for mining based
> on fee rate (it's quick and good enough), then it seems like fee rate
> is the approximation that should ultimately prevail for txn
> replacement.
>
> [0]:
> https://github.com/bitcoin/bitcoin/blob/master/src/node/miner.cpp#L310-L320
> [1]:
> https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.cpp#L1106
> [2]:
> https://github.com/bitcoin/bitcoin/blob/master/src/txmempool.cpp#L1138-L1144