Pooled mining

Pooled mining is a mining approach where multiple generating clients contribute to the generation of a block, and then split the block reward according the contributed processing power. Pooled mining effectively reduces the granularity of the block generation reward, spreading it out more smoothly over time.

Introduction

With increasing generation difficulty, mining with lower-performance devices can take a very long time before block generation, on average. For example, with a mining speed of 1000 Khps, at a difficulty of 14484 (which was in effect at the end of December, 2010), the average time to generate a block is almost 2 years.

To provide a more smooth incentive to lower-performance miners, several pooled miners, using different approaches, have been created. With a mining pool, a lot of different people contribute to generating a block, and the reward is then split among them according to their processing contribution. This way, instead of waiting for years to generate 50btc in a block, a smaller miner may get a fraction of a bitcoin on a more regular basis.

A share is awarded by the mining pool to the clients who present a valid proof of work of the same type as the proof of work that is used for creating blocks, but of lesser difficulty, so that it requires less time on average to generate.

Pooled mining approaches

The problem with pooled mining is that steps must be taken to prevent cheating by the clients and the server. Currently there are several different approaches used.

The slush approach

Bitcoin Pooled Mining (BPM), sometimes referred to as "slush's pool", follows a score-based method. Older shares (from beginning of the round) have lower weight than more recent shares, which reduces the motivation to cheat by switching between pools within a round.

The puddinpop approach

(As of February, 2011, there are no puddinpop pools running.)

Another approach is the 'metahash' technique, used by puddinpop's remote miner. Clients generate hashes, and also submit 'metahashes', which are hashes of a large chunk of generated hashes. The server checks that the metahashes are correct (in a round-robin fashion, picking up a metahash from a client that hasn't been checked on the longest), thus preventing clients from simply claiming that they have done work without actually doing it. The withholding of good blocks by the clients is prevented by the server's possession of the private key, just as in the previous approach. Rewards are distributed based on the number of metahashes submitted by the clients.

The generated blocks contain multiple keys in the generation transaction, giving fractional bitcoin amounts to each key in proportion to their hashing contribution for that block.

The Pay-per-Share approach

The Pay-per-Share (PPS) approach, first described by BitPenny, is to offer an instant flat payout for each share that is solved. The payout is offered from the pool's existing balance and can therefore be withdrawn immediately, without waiting for a block to be solved or confirmed. The possibility of cheating the miners by the pool operator and by timing attacks is thus completely eliminated.

This method results in the least possible variance for miners while transferring all risk to the pool operator. The resulting possibility of loss for the server is offset by setting a payout lower than the full expected value.

Luke-Jr's approach ("Eligius")

Luke came up with a third approach borrowing strengths from the earlier two. Like slush's approach, miners submit proofs-of-work to earn shares. Like puddinpop's approach, the pool pays out immediately via block generation. When distributing block rewards, it is divided equally among all shares since the last valid block. Unlike any preexisting pool approach, this means that the shares contributed toward orphaned blocks are recycled into the next block's shares. In order to spare participating miners from transaction fees, rewards are only paid out if a miner has earned at least 1 BTC. If the amount owed is less, it will be added to the earnings of a later block (which may then total over 1 BTC). If a miner does not submit a share for over a week, the pool sends any balance remaining, regardless of its size.

The Triplemining approach

The Triplemining approach is to bring together a medium-sized pool with no fees and clever redistribution of 1% of every found block to allow your share to grow more rapidly than on any other bitcoin mining pool.

For every found block, Triplemining redistributes 1% of the profits to all minipool owners (people with 1 or more friends mining with them). The redistribution is connected to the shares found by the members of the minipool. So if the hash rate of the minipool members equals or is bigger than yours, the part in the redistribution will be equally bigger.

P2Pool approach

P2Pool mining nodes work on a chain of shares similar to Bitcoin’s blockchain. There is no central point of failure and thus P2Pool becomes DoS resistant.

P2Pool works different from existing mining pool technologies — each node works on a block that includes payouts to the previous shares’ owners and the node itself. 99% of the block reward (the 50BTC reward plus any included transaction fees) is distributed evenly to miners based on work done recently. An additional 0.5% is awarded to the node which solves the block.

Comparison

The cooperative mining approach (slush and Luke-Jr) uses a lot less resources on the pool server, since rather than continuously checking metahashes, all that has to be checked is the validity of submitted shares. The number of shares sent can be adjusted by adjusting the artificial difficulty level.

Further, the cooperative mining approach allows the clients to use existing miners without any modification, while the puddinpop approach requires the custom pool miner, which are as of now not as efficient on GPU mining as the existing GPU miners.

Puddinpop miners receive coins directly.

Additionally, the puddinpop and Luke-Jr approaches of distributing the earnings by way of including precise sub-cent amounts in the generation transaction for the participants, results in the presence of sub-cent bitcoin amounts in your wallet, which are liable to disappear (as unnecessary fees) later due to a bug in old (before 0.3.21) bitcoin nodes. (E.g., if you have a transaction with 0.052 in your wallet, and you later send .05 to someone, your .002 will disappear.).

Puddinpop and Luke-Jr miners receive coins directly, which eliminates the delay in receiving earnings that is required on slush-based mining servers. However, using some eWallet services like MyBitcoin for generated coin will cause those coins to be lost.

See Also

• Miners
• Comparison of mining pools
• Pool Operators
• Why a GPU mines faster than a CPU
• Why pooled mining
• Mining pool reward FAQ

External links

• BTCSERV.net - Bitcoin Pool
• 21bitcoin mining pool, Chinese Interface
• slush's mining pool
• bitcash.cz - czech mining pool
• puddinpop's mining pool thread
• example puddinpop block
• bitclockers mining pool
• P2Pool forum