# max_difficulty (/docs/hyperparameters/max-difficulty)

`max_difficulty` is the ceiling over
[`difficulty`](/docs/hyperparameters/difficulty), the proof-of-work target
for PoW neuron registration. However hot registration demand runs, the
difficulty controller never pushes the PoW price above this value. At
`u64::MAX` the ceiling is effectively removed — difficulty is unbounded
above. Subnet owners tune it to cap how expensive a PoW slot can become
during registration rushes.

## How it works [#how-it-works]

Storage is `MaxDifficulty` in `pallets/subtensor/src/lib.rs`, accessed via
`get_max_difficulty` / `set_max_difficulty`
(`pallets/subtensor/src/utils/misc.rs`). In the classic controller,
difficulty was rescaled each adjustment interval by registration pressure
against the target rate, then clamped to the
\[`min_difficulty`, `max_difficulty`] band — sustained over-target demand
walked difficulty up until it hit this ceiling. The mainnet default is
`u64::MAX / 4` (`SubtensorInitialMaxDifficulty` in `runtime/src/lib.rs`).

On the current runtime PoW registration is deprecated — the `register`
extrinsic routes to burned registration and nothing adjusts `difficulty`
between intervals — so this ceiling is dormant. The playground opens on a
registration-rush scenario where difficulty ratchets up until it pins
against the bold ceiling line:

<HyperparamPowDifficulty focus="max_difficulty" />

## Reading and setting [#reading-and-setting]

```
btcli sudo get --netuid N --name max_difficulty
```

Owner-settable (the AdminUtils extrinsic `sudo_set_max_difficulty` accepts
the subnet owner or root, inside the admin window and subject to the owner
rate limit):

```
btcli sudo set --netuid N --name max_difficulty --value 4611686018427387903
```

The value is a raw u64 difficulty, no human form.

## Related [#related]

[`difficulty`](/docs/hyperparameters/difficulty) ·
[`min_difficulty`](/docs/hyperparameters/min-difficulty) ·
[`network_pow_registration_allowed`](/docs/hyperparameters/network-pow-registration-allowed) ·
[`target_regs_per_interval`](/docs/hyperparameters/target-regs-per-interval) ·
[`adjustment_interval`](/docs/hyperparameters/adjustment-interval) ·
[`max_burn`](/docs/hyperparameters/max-burn)