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@@ -0,0 +1,123 @@
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+---
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+simd: '0194'
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+title: Deprecate Rent Exemption Threshold
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+authors:
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+ - Dean Little (@deanmlittle)
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+ - Leonardo Donatacci (@L0STE)
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+ - febo (Anza)
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+category: Standard
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+type: Core
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+status: Accepted
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+created: 2024-11-13
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+feature: (fill in with feature tracking issues once accepted)
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+---
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+
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+## Summary
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+
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+This proposal aims to eliminate the use of floating-point operations to
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+determine whether an account is rent exempt or not in programs by deprecating
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+the use of the `exempt_threshold` (`f64`) value.
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+
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+More specifically, rename `lamports_per_byte_year` to `lamports_per_byte`,
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+change default value from `3480` to `6960`, change `exemption_threshold` to
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+`1.0` and deprecate it from the protocol. This will enable us to remove `f64`
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+math from all Rent-related SDKs and onchain programs moving forward.
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+
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+## Motivation
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+
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+Emulating floating point math is expensive inside of SVM due to the lack of
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+native floating point number support. This makes calculating the rent exempt
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+threshold cost `~248` more CUs than it would if we were to simply use a `u64`.
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+This is due to the `exemption_threshold` (`f64`), which is currently set to
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+`2.0` years. Since rent exemption is no longer time-based, we have the
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+opportunity to make this commonly used calculation much more performant and
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+simplify our tooling without affecting the existing API. It also simplifies any
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+further changes we may want to make to Rent down the line.
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+
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+## New Terminology
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+
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+`lamports_per_byte` - the number of lamports required to pay for 1 byte of
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+account storage.
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+
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+## Detailed Design
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+
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+Set the value of `DEFAULT_EXEMPTION_THRESHOLD` from `2.0` to `1.0`, and
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+deprecate it from the protocol.
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+
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+```rs
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+pub const DEFAULT_EXEMPTION_THRESHOLD: f64 = 1.0;
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+```
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+
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+Set `DEFAULT_LAMPORTS_PER_BYTE` from its current value of `3480` (the `u64`
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+value of `1_000_000_000 / 100 * 365 / (1024 * 1024)`), to `6960`; double its
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+current `u64` value, to counteract reducing the exemption threshold above.
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+
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+```rs
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+pub const DEFAULT_LAMPORTS_PER_BYTE: u64 = 6960;
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+```
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+
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+Rename `lamports_per_byte_year` in the Rent account to `lamports_per_byte` to
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+reflect that Rent is no longer time-based and officially deprecate
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+`exemption_threshold`.
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+
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+```rs
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+pub struct Rent {
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+ pub lamports_per_byte: u64,
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+ #[deprecated(since = "2.X.X", note = "Use only `lamports_per_byte`
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+instead")]
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+ pub exemption_threshold: f64,
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+ pub burn_percent: u8,
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+}
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+```
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+
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+ Remove all `f64` math from Rent calculations in all SDKs and onchain programs
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+moving forwards, replacing it with simple `u64` math, ie:
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+
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+```rs
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+/// Minimum balance due for rent-exemption of a given account data size.
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+pub fn minimum_balance(&self, data_len: usize) -> u64 {
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+ (ACCOUNT_STORAGE_OVERHEAD + data_len as u64) * self.lamports_per_byte
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+}
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+```
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+
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+Validator implementations should stop using `exemption_threshold` and only use
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+the `lamports_per_byte` value. Any existing program using the current
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+implementation will be unaffected.
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+
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+## Alternatives Considered
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+
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+- Leave things as they are.
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+ - Calculating rent exemption on a program remains an
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+expensive operation.
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+
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+- Allow users to make the assumption that `2.0` will remain stable and do `u64`
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+math themselves.
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+ - Risk of the protocol changing on them.
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+
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+- Perform the "conversion" ouside the VM and change the type of
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+`exemption_threshold` to a `u64` using a new `RentV2` struct.
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+ - Requires a new syscall and sysvar.
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+
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+## Impact
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+
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+This change will significantly improve the compute unit (CU) consumption of
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+new onchain programs using updated SDKs programs that calculate the minimum
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+lamports balance required for rent exemption. Currently, the calculation of the
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+minimum balance for rent exemption (`Rent::minimum_balance`) consumes
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+approximately `256` CUs; with the proposed change, this will be reduced to
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+just `8` CUs. Existing programs will not be impacted.
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+
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+## Security Considerations
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+
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+None.
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+
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+## Drawbacks
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+
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+None.
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+
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+## Backwards Compatibility
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+
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+This feature is backwards compatible. It does not change the absolute value
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+required for an account to be rent exempt and the current way of calculating
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+the threshold will continue to evaluate to the same value. Any deployed program
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+will be unaffected.
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Dean 利迪恩