[bitcoin-dev] Capacity increases for the Bitcoin system.

Anthony Towns aj at erisian.com.au
Tue Dec 8 06:54:48 UTC 2015


On Tue, Dec 08, 2015 at 05:21:18AM +0000, Gregory Maxwell via bitcoin-dev wrote:
> On Tue, Dec 8, 2015 at 4:58 AM, Anthony Towns via bitcoin-dev
> <bitcoin-dev at lists.linuxfoundation.org> wrote:
> > Having a cost function rather than separate limits does make it easier to
> > build blocks (approximately) optimally, though (ie, just divide the fee by
> > (base_bytes+witness_bytes/4) and sort). Are there any other benefits?
> Actually being able to compute fees for your transaction: If there are
> multiple limits that are "at play" then how you need to pay would
> depend on the entire set of other candidate transactions, which is
> unknown to you.

Isn't that solvable in the short term, if miners just agree to order
transactions via a cost function, without enforcing it at consensus
level until a later hard fork that can also change the existing limits
to enforce that balance?

(1MB base + 3MB witness + 20k sigops) with segwit initially, to something
like (B + W + 200*U + 40*S < 5e6) where B is base bytes, W is witness
bytes, U is number of UTXOs added (or removed) and S is number of sigops,
or whatever factors actually make sense.

I guess segwit does allow soft-forking more sigops immediately -- segwit
transactions only add sigops into the segregated witness, which doesn't
get counted for existing consensus. So it would be possible to take the
opposite approach, and make the rule immediately be something like:

  50*S < 1M
  B + W/4 + 25*S' < 1M

(where S is sigops in base data, and S' is sigops in witness) and
just rely on S trending to zero (or soft-fork in a requirement that
non-segregated witness transactions have fewer than B/50 sigops) so that
there's only one (linear) equation to optimise, when deciding fees or
creating a block. (I don't see how you could safely set the coefficient
for S' too much smaller though)

B+W/4+25*S' for a 2-in/2-out p2pkh would still be 178+206/4+25*2=280
though, which would allow 3570 transactions per block, versus 2700 now,
which would only be a 32% increase...

> These don't, however, apply all that strongly if only one limit is
> likely to be the limiting limit... though I am unsure about counting
> on that; after all if the other limits wouldn't be limiting, why have
> them?

Sure, but, at least for now, there's already two limits that are being
hit. Having one is *much* better than two, but I don't think two is a
lot better than three?

(Also, the ratio between the parameters doesn't necessary seem like a
constant; it's not clear to me that hardcoding a formula with a single
limit is actually better than hardcoding separate limits, and letting
miners/the market work out coefficients that match the sort of contracts
that are actually being used)

> > That seems kinda backwards.
> It can seem that way, but all limiting schemes have pathological cases
> where someone runs up against the limit in the most costly way. Keep
> in mind that casual pathological behavior can be suppressed via
> IsStandard like rules without baking them into consensus; so long as
> the candidate attacker isn't miners themselves. Doing so where
> possible can help avoid cases like the current sigops limiting which
> is just ... pretty broken.

Sure; it just seems to be halving the increase in block space (60% versus
100% extra for p2pkh, 100% versus 200% for 2/2 multisig p2sh) for what
doesn't actually look like that much of a benefit in fee comparisons?

I mean, as far as I'm concerned, segwit is great even if it doesn't buy
any improvement in transactions/block, so even a 1% gain is brilliant.
I'd just rather the 100%-200% gain I was expecting. :)

Cheers,
aj


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