Ommer Blocks

• From this blog by 0x (17-9-2020):

"Throughout its history, Ethereum never uses more than about 95% of the limit despite demand being there. Surprisingly this remaining 5% is wasted in blocks that are completely empty. These empty blocks happen regularly, about once every twenty blocks. Why would someone mine empty blocks when there are transactions that will pay for inclusion?

The rate of empty blocks is steadily increasing over time, and currently at about 5%. All the mining pools contribute to it equally, so it’s not a malicious miner. Instead, the real causes seem to be fast blocks. If the time to mine a block was less than 6 seconds, the chances of it being empty go up exponentially.

One explanation is that miners start mining the next block the moment they receive a new block header, but before they have processed the whole block. This trick, known as SPV mining in Bitcoin, helps miners get started on the next block immediately, but it can only produce empty blocks. Once the new block is fully processed, the miner can assemble the next full block and switch to mining that one. Further evidence for this explanation is the empty block rate reduces by 25% if the same miner finds two blocks in quick succession.

The alternative to mining empty blocks is to keep mining on the previous block while you are processing the new one. This can cause multiple next blocks to be mined. If this happens Ethereum will pick a canonical block and remember the others as ommers. Miners still get a small reward for the ommers.

The ommer rate peaked during the 2018 usage spike but has been decreasing to a stable rate of 5% of all blocks mined. This coincides with the increase in the empty block rate as miners likely switched strategies around this time.

It is not immediately obvious that the ommer rate hurts Ethereum scaling but it does. Since EIP-100 in Byzantium the difficulty adjustment maintains a constant rate of canonical and ommer blocks. So a high ommer rate means more blocks are wasted on ommers and fewer grow the chain. This shows up as an increase in block times and thus less total gas available per day. (The other major cause for increased block times being ice ages.)

Whether it’s ommers or empty blocks, these are important network health signals for Ethereum. An increase in either means that less daily total gas is available for transactions. Analysis of the ommer rate is the big part of the research backing EIPs 2028 and 1559. It is surprising though that neither mentions the empty block rate and that the research has methodological flaws. It would be preferable to have a more rigourous analysis considering both ommer and empty rate using appropriate statistical methods like logistic regression.

There are ways to lower the ommer and empty rate. The presumed root cause is mining pools not having the latest state available due to network and processing delays. One simple but undesirable solution is to centralizing mining pools more so that latest state is in one place. Slightly more decentralized solutions like bloxroute are creating dedicated interconnects between pools. An idea inspired by ‘spy mining’ would be for pools to pre-share the block that they are currently trying to mine. Other pools would prepare next blocks for each eventuality and once a pool successfully mines its block, they already know which block they want to mine on next and can switch in an instant. Going higher up the stack, improvements in node communication protocols and processing algorithms would help too, there is likely still some room for gains here. But in the end, lowering the ommer and empty rate by itself will improve the daily gas supply by at most about the 5% we saw."