Deprecate Rent Exemption Threshold

Summary

This proposal aims to eliminate the use of floating-point operations to determine whether an account is rent exempt or not in programs by deprecating the use of the exempt_threshold (f64) value.

More specifically, rename lamports_per_byte_year to lamports_per_byte, change default value from 3480 to 6960, change exemption_threshold to 1.0 and deprecate it from the protocol. This will enable us to remove f64 math from all Rent-related SDKs and onchain programs moving forward.

To align the cluster value of burn_percent with current SDK defaults, we will also explicity set burn_percent to 50.

Motivation

Emulating floating point math is expensive inside of SVM due to the lack of native floating point number support. This makes calculating the rent exempt threshold cost ~248 more CUs than it would if we were to simply use a u64. This is due to the exemption_threshold (f64), which is currently set to 2.0 years. Since rent exemption is no longer time-based, we have the opportunity to make this commonly used calculation much more performant and simplify our tooling without affecting the existing API. It also simplifies any further changes we may want to make to Rent down the line.

New Terminology

lamports_per_byte - the number of lamports required to pay for 1 byte of account storage.

Detailed Design

Set the value of DEFAULT_EXEMPTION_THRESHOLD from 2.0 to 1.0, and deprecate it from the protocol.

pub const DEFAULT_EXEMPTION_THRESHOLD: f64 = 1.0;

Set DEFAULT_LAMPORTS_PER_BYTE from its current value of 3480 (the u64 value of 1_000_000_000 / 100 * 365 / (1024 * 1024)), to 6960; double its current u64 value, to counteract reducing the exemption threshold above.

pub const DEFAULT_LAMPORTS_PER_BYTE: u64 = 6960;

Rename lamports_per_byte_year in the Rent account to lamports_per_byte to reflect that Rent is no longer time-based and officially deprecate exemption_threshold.

pub struct Rent {
    pub lamports_per_byte: u64,
    #[deprecated(since = "2.X.X", note = "Use only `lamports_per_byte` 
instead")]
    pub exemption_threshold: f64,
    pub burn_percent: u8,
}

Remove all f64 math from Rent calculations in all SDKs and onchain programs moving forwards, replacing it with simple u64 math, ie:

/// Minimum balance due for rent-exemption of a given account data size.
pub fn minimum_balance(&self, data_len: usize) -> u64 {
    (ACCOUNT_STORAGE_OVERHEAD + data_len as u64) * self.lamports_per_byte
}

Validator implementations should stop using exemption_threshold and only use the lamports_per_byte value. Any existing program using the current implementation will be unaffected.

Validator implementations should also set burn_percent to 50 to align all cluster values to the current value of DEFAULT_BURN_PERCENT.

Alternatives Considered

• Leave things as they are. - Calculating rent exemption on a program remains an expensive operation.
• Allow users to make the assumption that 2.0 will remain stable and do u64 math themselves. - Risk of the protocol changing on them.
• Perform the "conversion" ouside the VM and change the type of exemption_threshold to a u64 using a new RentV2 struct. - Requires a new syscall and sysvar.

Impact

This change will significantly improve the compute unit (CU) consumption of new onchain programs using updated SDKs programs that calculate the minimum lamports balance required for rent exemption. Currently, the calculation of the minimum balance for rent exemption (Rent::minimum_balance) consumes approximately 256 CUs; with the proposed change, this will be reduced to just 8 CUs. Existing programs will not be impacted.

Security Considerations

None.

Drawbacks

None.

Backwards Compatibility

This feature is backwards compatible. It does not change the absolute value required for an account to be rent exempt and the current way of calculating the threshold will continue to evaluate to the same value. Any deployed program will be unaffected.