From: Dave Hudson <dave@hashingit•com>
To: Bob McElrath <bob_bitcoin@mcelrath•org>
Cc: bitcoin-dev@lists•linuxfoundation.org
Subject: Re: [bitcoin-dev] Hardfork to fix difficulty drop algorithm
Date: Wed, 9 Mar 2016 23:24:15 +0000 [thread overview]
Message-ID: <DD5ED99B-6127-4D9F-9E39-B623234281BA@hashingit.com> (raw)
In-Reply-To: <20160309202135.GC4388@mcelrath.org>
> On 9 Mar 2016, at 20:21, Bob McElrath <bob_bitcoin@mcelrath•org> wrote:
>
> Dave Hudson [dave@hashingit•com] wrote:
>> A damping-based design would seem like the obvious choice (I can think of a
>> few variations on a theme here, but most are found in the realms of control
>> theory somewhere). The problem, though, is working working out a timeframe
>> over which to run the derivative calculations.
>
> From a measurement theory perspective this is straightforward. Each block is a
> measurement, and error propagation can be performed to derive an error on the
> derivatives.
Sure, but I think there are 2 problems:
1) My guess is that errors over anything but a long period are probably too large to be very useful.
2) We don't have a strong notion of time that is part of the consensus. Sure, blocks have timestamps but they're very loosely controlled (can't be more than 2 hours ahead of what any validating node thinks the time might be). Difficulty can't be calculated based on anything that's not part of the consensus data.
> The statistical theory of Bitcoin's block timing is known as a Poisson Point
> Process: https://en.wikipedia.org/wiki/Poisson_point_process or temporal point
> process. If you google those plus "estimation" you'll find a metric shit-ton of
> literature on how to handle this.
Strictly it's a non-homogeneous Poisson Process, but I'm pretty familiar with the concept (Google threw one of my own blog posts back at me: http://hashingit.com/analysis/27-hash-rate-headaches, but I actually prefer this one: http://hashingit.com/analysis/30-finding-2016-blocks because most people seem to find it easier to visualize).
>> The problem is the measurement of the hashrate, which is pretty inaccurate at
>> best because even 2016 events isn't really enough (with a completely constant
>> hash rate running indefinitely we'd see difficulty swings of up to +/- 5% even
>> with the current algorithm). In order to meaningfully react to a major loss
>> of hashing we'd still need to be considering a window of probably 2 weeks.
>
> You don't want to assume it's constant in order to get a better measurement.
> The assumption is clearly false. But, errors can be calculated, and retargeting
> can take errors into account, because no matter what we'll always be dealing
> with a finite sample.
Agreed, it's a thought experiment I ran in May 2014 (http://hashingit.com/analysis/28-reach-for-the-ear-defenders). I found that many people's intuition is that there would be little or no difficulty changes in such a scenario, but the intuition isn't reliable. Given a static hash rate the NHPP behaviour introduces a surprisingly large amount of noise (often much larger than any signal over a period of even weeks). Any measurements in the order of even a few days has so much noise that it's practically unusable. I just realized that unlike some of my other sims this one didn't make it to github; I'll fix that later this week.
Cheers,
Dave
next prev parent reply other threads:[~2016-03-09 23:24 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-03-02 14:56 Luke Dashjr
2016-03-02 15:05 ` Pavel Janík
2016-03-02 15:14 ` Luke Dashjr
2016-03-02 15:24 ` Jérémie Dubois-Lacoste
[not found] ` <CAE-z3OUR8So2EM_EBeEerW-UPs0KY+whVB=jjFAHkW3xZPF2Hw@mail.gmail.com>
2016-03-02 15:54 ` Tier Nolan
2016-03-02 15:42 ` Luke Dashjr
2016-03-02 16:27 ` Paul Sztorc
2016-03-02 18:07 ` Tier Nolan
2016-03-02 19:01 ` Eric Voskuil
[not found] ` <56D74859.3090609@gmail.com>
2016-03-02 20:44 ` Eric Voskuil
2016-03-02 23:02 ` Peter Todd
2016-03-03 5:11 ` Dave Scotese
2016-03-03 10:14 ` Patrick Shirkey
2016-03-03 20:54 ` Eric Voskuil
2016-03-04 10:27 ` Tier Nolan
2016-03-02 15:48 ` Dave Hudson
2016-03-08 22:05 ` Bob McElrath
2016-03-09 18:30 ` Dave Hudson
2016-03-09 20:21 ` Bob McElrath
2016-03-09 23:24 ` Dave Hudson [this message]
2016-03-09 20:26 ` Paul Sztorc
2016-03-02 16:17 ` Bryan Bishop
2016-03-02 17:14 ` David A. Harding
2016-03-02 17:53 ` Gregory Maxwell
2016-03-02 19:34 ` David A. Harding
2016-03-03 1:06 ` Paul Sztorc
2016-03-09 17:58 ` Paul Sztorc
2016-03-02 18:20 ` Peter Todd
2016-03-03 18:27 ` Corey Haddad
2016-03-04 8:41 ` Henning Kopp
[not found] ` <CA+XQW1gfnXxxCod6cL=caGnEc66YOvaF6SJL=omUbMqwLNDP7g@mail.gmail.com>
2016-03-09 20:43 ` Paul Sztorc
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