[syscalls] Update mod exp compute cost (#2807)

anza-xyz · Sep 2, 2024 · eb37b21 · eb37b21
1 parent 4659342
commit eb37b21
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Showing 2 changed files with 16 additions and 7 deletions.
diff --git a/compute-budget/src/compute_budget.rs b/compute-budget/src/compute_budget.rs
@@ -103,8 +103,12 @@ pub struct ComputeBudget {
     /// + alt_bn128_pairing_one_pair_cost_other * (num_elems - 1)
     pub alt_bn128_pairing_one_pair_cost_first: u64,
     pub alt_bn128_pairing_one_pair_cost_other: u64,
-    /// Big integer modular exponentiation cost
-    pub big_modular_exponentiation_cost: u64,
+    /// Big integer modular exponentiation base cost
+    pub big_modular_exponentiation_base_cost: u64,
+    /// Big integer moduler exponentiation cost divisor
+    /// The modular exponentiation cost is computed as
+    /// `input_length`/`big_modular_exponentiation_cost_divisor` + `big_modular_exponentiation_base_cost`
+    pub big_modular_exponentiation_cost_divisor: u64,
     /// Coefficient `a` of the quadratic function which determines the number
     /// of compute units consumed to call poseidon syscall for a given number
     /// of inputs.
@@ -180,7 +184,8 @@ impl ComputeBudget {
             alt_bn128_multiplication_cost: 3_840,
             alt_bn128_pairing_one_pair_cost_first: 36_364,
             alt_bn128_pairing_one_pair_cost_other: 12_121,
-            big_modular_exponentiation_cost: 33,
+            big_modular_exponentiation_base_cost: 190,
+            big_modular_exponentiation_cost_divisor: 2,
             poseidon_cost_coefficient_a: 61,
             poseidon_cost_coefficient_c: 542,
             get_remaining_compute_units_cost: 100,

diff --git a/programs/bpf_loader/src/syscalls/mod.rs b/programs/bpf_loader/src/syscalls/mod.rs
@@ -1710,13 +1710,15 @@ declare_builtin_function!(
         let input_len: u64 = std::cmp::max(input_len, params.modulus_len);
 
         let budget = invoke_context.get_compute_budget();
+        // the compute units are calculated by the quadratic equation `0.5 input_len^2 + 190`
         consume_compute_meter(
             invoke_context,
             budget.syscall_base_cost.saturating_add(
                 input_len
                     .saturating_mul(input_len)
-                    .checked_div(budget.big_modular_exponentiation_cost)
-                    .unwrap_or(u64::MAX),
+                    .checked_div(budget.big_modular_exponentiation_cost_divisor)
+                    .unwrap_or(u64::MAX)
+                    .saturating_add(budget.big_modular_exponentiation_base_cost),
             ),
         )?;
 
@@ -4719,7 +4721,8 @@ mod tests {
             let budget = invoke_context.get_compute_budget();
             invoke_context.mock_set_remaining(
                 budget.syscall_base_cost
-                    + (MAX_LEN * MAX_LEN) / budget.big_modular_exponentiation_cost,
+                    + (MAX_LEN * MAX_LEN) / budget.big_modular_exponentiation_cost_divisor
+                    + budget.big_modular_exponentiation_base_cost,
             );
 
             let result = SyscallBigModExp::rust(
@@ -4760,7 +4763,8 @@ mod tests {
             let budget = invoke_context.get_compute_budget();
             invoke_context.mock_set_remaining(
                 budget.syscall_base_cost
-                    + (INV_LEN * INV_LEN) / budget.big_modular_exponentiation_cost,
+                    + (INV_LEN * INV_LEN) / budget.big_modular_exponentiation_cost_divisor
+                    + budget.big_modular_exponentiation_base_cost,
             );
 
             let result = SyscallBigModExp::rust(