On Fri, May 8, 2015 at 8:33 PM, Mark Friedenbach <m...@friedenbach.org> wrote: > These rules create an incentive environment where raising the block size has > a real cost associated with it: a more difficult hashcash target for the > same subsidy reward. For rational miners that cost must be counter-balanced > by additional fees provided in the larger block. This allows block size to > increase, but only within the confines of a self-supporting fee economy. > > When the subsidy goes away or is reduced to an insignificant fraction of the > block reward, this incentive structure goes away. Hopefully at that time we > would have sufficient information to soft-fork set a hard block size > maximum. But in the mean time, the block size limit controller constrains > the maximum allowed block size to be within a range supported by fees on the > network, providing an emergency relief valve that we can be assured will > only be used at significant cost.
Though I'm a fan of this class of techniques(*) and think using something in this space is strictly superior to not, and I think it makes larger sizes safer long term; I do not think it adequately obviates the need for a hard upper limit for two reasons: (1) for software engineering and operational reasons it is very difficult to develop, test for, or provision for something without knowing limits. There would in fact be hard limits on real deployments but they'd be opaque to their operators and you could easily imagine the network forking by surprise as hosts crossed those limits. (2) At best this approach mitigates the collective action problem between miners around fees; it does not correct the incentive alignment between miners and everyone else (miners can afford huge node costs because they have income; but the full-node-using-users that need to exist in plenty to keep miners honest do not), or the centralization pressures (N miners can reduce their storage/bandwidth/cpu costs N fold by centralizing). A dynamic limit can be combined with a hard upper to at least be no worse than a hard upper with respect to those two points. Another related point which has been tendered before but seems to have been ignored is that changing how the size limit is computed can help better align incentives and thus reduce risk. E.g. a major cost to the network is the UTXO impact of transactions, but since the limit is blind to UTXO impact a miner would gain less income if substantially factoring UTXO impact into its fee calculations; and without fee impact users have little reason to optimize their UTXO behavior. This can be corrected by augmenting the "size" used for limit calculations. An example would be tx_size = MAX( real_size >> 1, real_size + 4*utxo_created_size - 3*utxo_consumed_size). The reason for the MAX is so that a block which cleaned a bunch of big UTXO could not break software by being super large, the utxo_consumed basically lets you credit your fees by cleaning the utxo set; but since you get less credit than you cost the pressure should be downward but not hugely so. The 1/2, 4, 3 I regard as parameters which I don't have very strong opinions on which could be set based on observations in the network today (e.g. adjusted so that a normal cleaning transaction can hit the minimum size). One way to think about this is that it makes it so that every output you create "prepays" the transaction fees needed to spend it by shifting "space" from the current block to a future block. The fact that the prepayment is not perfectly efficient reduces the incentive for miners to create lots of extra outputs when they have room left in their block in order to store space to use later [an issue that is potentially less of a concern with a dynamic size limit]. With the right parameters there would never be such at thing as a dust output (one which costs more to spend than its worth). (likewise the sigops limit should be counted correctly and turned into size augmentation (ones that get run by the txn); which would greatly simplify selection rules: maximize income within a single scalar limit) (*) I believe my currently favored formulation of general dynamic control idea is that each miner expresses in their coinbase a preferred size between some minimum (e.g. 500k) and the miner's effective-maximum; the actual block size can be up to the effective maximum even if the preference is lower (you're not forced to make a lower block because you stated you wished the limit were lower). There is a computed maximum which is the 33-rd percentile of the last 2016 coinbase preferences minus computed_max/52 (rounding up to 1) bytes-- or 500k if thats larger. The effective maximum is X bytes more, where X on the range [0, computed_maximum] e.g. the miner can double the size of their block at most. If X > 0, then the miners must also reach a target F(x/computed_maximum) times the bits-difficulty; with F(x) = x^2+1 --- so the maximum penalty is 2, with a quadratic shape; for a given mempool there will be some value that maximizes expected income. (obviously all implemented with precise fixed point arithmetic). The percentile is intended to give the preferences of the 33% least preferring miners a veto on increases (unless a majority chooses to soft-fork them out). The minus-comp_max/52 provides an incentive to slowly shrink the maximum if its too large-- x/52 would halve the size in one year if miners were doing the lowest difficulty mining. The parameters 500k/33rd, -computed_max/52 bytes, and f(x) I have less strong opinions about; and would love to hear reasoned arguments for particular parameters. ------------------------------------------------------------------------------ One dashboard for servers and applications across Physical-Virtual-Cloud Widest out-of-the-box monitoring support with 50+ applications Performance metrics, stats and reports that give you Actionable Insights Deep dive visibility with transaction tracing using APM Insight. http://ad.doubleclick.net/ddm/clk/290420510;117567292;y _______________________________________________ Bitcoin-development mailing list Bitcoin-development@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/bitcoin-development
