[Bitcoin-development] Proposed alternatives to the 20MB step function
Gregory Maxwell
gmaxwell at gmail.com
Sat May 9 03:36:07 UTC 2015
On Fri, May 8, 2015 at 8:33 PM, Mark Friedenbach <mark at 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.
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