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From: Sjors Provoost <sjors@sprovoost.nl>
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Subject: Re: [bitcoindev] Unbreaking testnet4
Date: Wed, 19 Mar 2025 09:32:36 +0100
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Antoine Poinsot wrote:

> The given rationale for a difficulty reset was to let developers occasion=
ally mine blocks on their laptop. But you cannot have your cake and eat it =
too: either the network is permissionless (PoW) or you assign identities an=
d privileges to some (Signet). By trying to do both at the same time testne=
t4 created a loophole for abuse. As a result it failed on both count: it ne=
ither mimics mainnet nor allows developers to mine active blocks on their l=
aptop.

Let me clarify why developers can't mine on their own laptop with testnet4.

The way you would do that is to move your computer clock forward by 20 minu=
tes (or use faketime), at which point the difficulty drops to 1. You would =
then mine your (presumably non-standard transaction) and broadcast it to th=
e network.

This is also the strategy used to acquire more testnet coins if you don't h=
ave an ASIC and don't want to use a faucet.

On testnet3 the latter wasn't very productive because at height 4 million t=
he block subsidy is under 10k sats.

But on testnet4 every block yields 50 tBTC. So several people try to mine s=
uch a block, leading to the many parallel forks.=20

If you're a developer trying to mine a non-standard transaction, you have t=
o be fast, well connected and lucky to be the first block picked up by the =
rest of the network.

But why mine just one if you can mine many?

Some CPU miners are now mining as many testnet4 blocks as they can by bumpi=
ng their clock 20 minutes not just once, but several times in a row. Until =
they run against the limit other nodes put on how far a block can be in the=
 future, namely two hours. So when a real difficulty block appears, you may=
 see 5 blocks on top of it instantly.

This behavior is even worse from the point of view of a developer trying to=
 mine a non-standard transaction. Because the tip of your node is always go=
ing to be about two hours in the future, when you mine on top of that by mo=
ving your clock even further, it will be rejected by your peers.

So this use case of CPU mining non-standard transactions is simply dead as =
long as this behaviour exists. We might as well reduce code complexity.

That said, testnet will never mimic mainnet. It has no value, or worse, ver=
y little value. So the incentives are different, which leads to different b=
ehavior. That's a whack-a-mole game, which we should probably not dedicate =
time to.

- Sjors


=20

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<html><head><meta http-equiv=3D"content-type" content=3D"text/html; charset=
=3Dus-ascii"></head><body style=3D"overflow-wrap: break-word; -webkit-nbsp-=
mode: space; line-break: after-white-space;">Antoine Poinsot wrote:<div><br=
><div><blockquote type=3D"cite"><div><div style=3D"font-family: Arial, sans=
-serif; font-size: 14px;"><div style=3D"font-family: Arial, sans-serif; fon=
t-size: 14px;"><div>The
 given rationale for a difficulty reset was to let developers=20
occasionally mine blocks on their laptop. But you cannot have your cake=20
and eat it too: either the network is permissionless (PoW) or you assign
 identities and privileges to some (Signet). By trying to do both at the
 same time testnet4 created a loophole for abuse. As a result it failed=20
on both count: it neither mimics mainnet nor allows developers to mine=20
active blocks on their laptop.<br></div></div></div></div></blockquote><br>=
</div><div>Let me clarify why developers can't mine on their own laptop wit=
h testnet4.</div><div><br></div><div>The way you would do that is to move y=
our computer clock forward by 20 minutes (or use faketime), at which point =
the difficulty drops to 1. You would then mine your (presumably non-standar=
d transaction) and broadcast it to the network.</div><div><br></div><div>Th=
is is also the strategy used to acquire more testnet coins if you don't hav=
e an ASIC and don't want to use a faucet.</div><div><br></div></div><div>On=
 testnet3 the latter wasn't very productive because at height 4 million the=
 block subsidy is under 10k sats.</div><div><br></div><div>But on testnet4 =
every block yields 50 tBTC. So several people try to mine such a block, lea=
ding to the many parallel forks.&nbsp;</div><div><br></div><div>If you're a=
 developer trying to mine a non-standard transaction, you have to be fast, =
well connected and lucky to be the first block picked up by the rest of the=
 network.</div><div><br></div><div>But why mine just one if you can mine ma=
ny?</div><div><br></div><div>Some CPU miners are now mining as many testnet=
4 blocks as they can by bumping their clock 20 minutes not just once, but s=
everal times in a row. Until they run against the limit other nodes put on =
how far a block can be in the future, namely two hours. So when a real diff=
iculty block appears, you may see 5 blocks on top of it instantly.</div><di=
v><br></div><div>This behavior is even worse from the point of view of a de=
veloper trying to mine a non-standard transaction. Because the tip of your =
node is always going to be about two hours in the future, when you mine on =
top of that by moving your clock even further, it will be rejected by your =
peers.</div><div><br></div><div>So this use case of CPU mining non-standard=
 transactions is simply dead as long as this behaviour exists. We might as =
well reduce code complexity.</div><div><br></div><div>That said, testnet wi=
ll never mimic mainnet. It has no value, or worse, very little value. So th=
e incentives are different, which leads to different behavior. That's a wha=
ck-a-mole game, which we should probably not dedicate time to.</div><div><b=
r></div><div>- Sjors</div><div><br></div><div><br></div><div>&nbsp;</div></=
body></html>

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