[Bitcoin-development] Proposed alternatives to the 20MB step function

Aaron Voisine voisine at gmail.com
Fri May 29 17:45:39 UTC 2015

> miners would definitely be squeezing out transactions / putting pressure
to increase transaction fees

I'd just like to re-iterate that transactions getting "squeezed out"
(failure after a lengthy period of uncertainty) is a radical change from
the current behavior of the network. There are plenty of avenues to create
fee pressure without resorting to such a drastic change in how the network
works today.

Aaron Voisine
co-founder and CEO

On Thu, May 28, 2015 at 8:53 AM, Gavin Andresen <gavinandresen at gmail.com>

> On Fri, May 8, 2015 at 3:20 AM, Matt Whitlock <bip at mattwhitlock.name>
> wrote:
>> Between all the flames on this list, several ideas were raised that did
>> not get much attention. I hereby resubmit these ideas for consideration and
>> discussion.
>> - Perhaps the hard block size limit should be a function of the actual
>> block sizes over some trailing sampling period. For example, take the
>> median block size among the most recent 2016 blocks and multiply it by 1.5.
>> This allows Bitcoin to scale up gradually and organically, rather than
>> having human beings guessing at what is an appropriate limit.
> A lot of people like this idea, or something like it. It is nice and
> simple, which is really important for consensus-critical code.
> With this rule in place, I believe there would be more "fee pressure"
> (miners would be creating smaller blocks) today. I created a couple of
> histograms of block sizes to infer what policy miners are ACTUALLY
> following today with respect to block size:
> Last 1,000 blocks:
>   http://bitcoincore.org/~gavin/sizes_last1000.html
> Notice a big spike at 750K -- the default size for Bitcoin Core.
> This graph might be misleading, because transaction volume or fees might
> not be high enough over the last few days to fill blocks to whatever limit
> miners are willing to mine.
> So I graphed a time when (according to statoshi.info) there WERE a lot of
> transactions waiting to be confirmed:
>    http://bitcoincore.org/~gavin/sizes_357511.html
> That might also be misleading, because it is possible there were a lot of
> transactions waiting to be confirmed because miners who choose to create
> small blocks got lucky and found more blocks than normal.  In fact, it
> looks like that is what happened: more smaller-than-normal blocks were
> found, and the memory pool backed up.
> So: what if we had a dynamic maximum size limit based on recent history?
> The average block size is about 400K, so a 1.5x rule would make the max
> block size 600K; miners would definitely be squeezing out transactions /
> putting pressure to increase transaction fees. Even a 2x rule (implying
> 800K max blocks) would, today, be squeezing out transactions / putting
> pressure to increase fees.
> Using a median size instead of an average means the size can increase or
> decrease more quickly. For example, imagine the rule is "median of last
> 2016 blocks" and 49% of miners are producing 0-size blocks and 51% are
> producing max-size blocks. The median is max-size, so the 51% have total
> control over making blocks bigger.  Swap the roles, and the median is
> min-size.
> Because of that, I think using an average is better-- it means the max
> size will change (up or down) more slowly.
> I also think 2016 blocks is too long, because transaction volumes change
> quicker than that. An average over 144 blocks (last 24 hours) would be
> better able to handle increased transaction volume around major holidays,
> and would also be able to react more quickly if an economically irrational
> attacker attempted to flood the network with fee-paying transactions.
> So my straw-man proposal would be:  max size 2x average size over last 144
> blocks, calculated at every block.
> There are a couple of other changes I'd pair with that consensus change:
> + Make the default mining policy for Bitcoin Core neutral-- have its
> target block size be the average size, so miners that don't care will "go
> along with the people who do care."
> + Use something like Greg's formula for size instead of bytes-on-the-wire,
> to discourage bloating the UTXO set.
> ---------
> When I've proposed (privately, to the other core committers) some dynamic
> algorithm the objection has been "but that gives miners complete control
> over the max block size."
> I think that worry is unjustified right now-- certainly, until we have
> size-independent new block propagation there is an incentive for miners to
> keep their blocks small, and we see miners creating small blocks even when
> there are fee-paying transactions waiting to be confirmed.
> I don't even think it will be a problem if/when we do have
> size-independent new block propagation, because I think the combination of
> the random timing of block-finding plus a dynamic limit as described above
> will create a healthy system.
> If I'm wrong, then it seems to me the miners will have a very strong
> incentive to, collectively, impose whatever rules are necessary (maybe a
> soft-fork to put a hard cap on block size) to make the system healthy again.
> --
> --
> Gavin Andresen
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