  Mining pools, the main components of the Bitcoin network, dominate the
computing resources and play essential roles in network security and
performance aspects. Although many existing measurements of the Bitcoin network
are available, little is known about the details of mining pool behaviors
(e.g., empty blocks, mining revenue and transaction collection strategies) and
their effects on the Bitcoin end users (e.g., transaction fees, transaction
delay and transaction acceptance rate). This paper aims to fill this gap with a
systematic study of mining pools. We traced over 1.56 hundred thousand blocks
(including about 257 million historical transactions) from February 2016 to
January 2019 and collected over 120.25 million unconfirmed transactions from
March 2018 to January 2019. Then we conducted a board range of measurements on
the pool evolutions, labeled transactions (blocks) as well as real-time network
traffics, and discovered new interesting observations and features.
Specifically, our measurements show the following. 1) A few mining pools
entities continuously control most of the computing resources of the Bitcoin
network. 2) Mining pools are caught in a prisoner's dilemma where mining pools
compete to increase their computing resources even though the unit profit of
the computing resource decreases. 3) Mining pools are stuck in a Malthusian
trap where there is a stage at which the Bitcoin incentives are inadequate for
feeding the exponential growth of the computing resources. 4) The market price
and transaction fees are not sensitive to the event of halving block rewards.
5) The block interval of empty blocks is significantly lower than the block
interval of non-empty blocks. 6) Feerate plays a dominating role in transaction
collection strategy for the top mining pools. Our measurements and analysis
help to understand and improve the Bitcoin network.
