Reduce lamports_per_byte to 3480

Summary

Reduce lamports_per_byte by 50% to 3480, which is equivalent to reducing the rent-exempt minimum balance by half for all new and existing accounts.

Requires SIMD-0392: Relaxation of post-execution min_balance check, to allow for subsequent rent increases if necessary.

Motivation

Minimum account balance depends on an arbitrary constant set years ago that has since increased significantly in real terms due to increases in the SOL price. This has made state allocation cost more expensive on mainnet beta than on competitors, with no compelling justification provided by real resource costs.

New Terminology

N/A

Detailed Design

On feature activation, set the effective lamports_per_byte to 3480 in the bank and rent sysvar (SysvarRent111111111111111111111111111111111), followed by updating DEFAULT_LAMPORTS_PER_BYTE in all relevant SDKs post-activation.

ACCOUNT_STORAGE_OVERHEAD = 128
lamports_per_byte = 3480 // reduced from 6960 by this proposal
effective_size = ACCOUNT_STORAGE_OVERHEAD + data_size_bytes
min_balance = effective_size * lamports_per_byte
            = effective_size * 3480

Alternatives Considered

This proposal is the first step towards further rent reduction and doesn't prevent adoption of alternative solutions like a supervisory controller (SIMD-0389). A 2x reduction is picking the low hanging fruit by modestly reducing the extremely inflated rent cost on mainnet beta today.

Impact

• Lower rent for app developers. Existing accounts and programs using the higher rent value will be unaffected, besides being allowed to reduce the balance to the new minimum.
• Validators: a potential increase in state growth. In the unlikely case of excessive state growth, rent can be increased back to the legacy value (0392 allows this without significant disruption to existing accounts.)

Security Considerations

N/A

Backwards Compatibility

Rent reduction is strictly a relaxation of existing constraints so all existing program logic will continue to work as before.