Deal with negative feed back loop in DA gas price

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ### Added
 - [2321](https://github.com/FuelLabs/fuel-core/pull/2321): New metrics for the txpool: "The size of transactions in the txpool" (`txpool_tx_size`), "The time spent by a transaction in the txpool in seconds" (`txpool_tx_time_in_txpool_seconds`), The number of transactions in the txpool (`txpool_number_of_transactions`), "The number of transactions pending verification before entering the txpool" (`txpool_number_of_transactions_pending_verification`), "The number of executable transactions in the txpool" (`txpool_number_of_executable_transactions`), "The time it took to select transactions for inclusion in a block in nanoseconds" (`txpool_select_transaction_time_nanoseconds`), The time it took to insert a transaction in the txpool in milliseconds (`txpool_insert_transaction_time_milliseconds`).
+- [2347](https://github.com/FuelLabs/fuel-core/pull/2364): Add activity concept in order to protect against infinitely increasing DA gas price scenarios
 
 ## [Version 0.40.0]
 

crates/fuel-gas-price-algorithm/gas-price-analysis/src/simulation.rs

@@ -1,5 +1,8 @@
 use super::*;
-use fuel_gas_price_algorithm::v1::AlgorithmUpdaterV1;
+use fuel_gas_price_algorithm::v1::{
+    AlgorithmUpdaterV1,
+    L2ActivityTracker,
+};
 use std::{
     collections::BTreeMap,
     num::NonZeroU64,
@@ -83,6 +86,7 @@ impl Simulator {
     ) -> AlgorithmUpdaterV1 {
         // Scales the gas price internally, value is arbitrary
         let gas_price_factor = 100;
+        let always_normal_activity = L2ActivityTracker::new_always_normal();
         let updater = AlgorithmUpdaterV1 {
             min_exec_gas_price: 10,
             min_da_gas_price: 10,
@@ -109,6 +113,7 @@ impl Simulator {
             da_d_component,
             last_profit: 0,
             second_to_last_profit: 0,
+            l2_activity: always_normal_activity,
         };
         updater
     }

crates/fuel-gas-price-algorithm/src/v1.rs

@@ -118,13 +118,8 @@ pub struct AlgorithmUpdaterV1 {
     /// This is a percentage of the total capacity of the L2 block
     pub l2_block_fullness_threshold_percent: ClampedPercentage,
     // DA
-    /// The gas price for the DA portion of the last block. This can be used to calculate
-    /// the DA portion of the next block
-    // pub last_da_gas_price: u64,
-
     /// The gas price (scaled by the `gas_price_factor`) to cover the DA commitment of the next block
     pub new_scaled_da_gas_price: u64,
-
     /// Scale factor for the gas price.
     pub gas_price_factor: NonZeroU64,
     /// The lowest the algorithm allows the da gas price to go
@@ -151,13 +146,134 @@ pub struct AlgorithmUpdaterV1 {
     pub second_to_last_profit: i128,
     /// The latest known cost per byte for recording blocks on the DA chain
     pub latest_da_cost_per_byte: u128,
-
+    /// Activity of L2
+    pub l2_activity: L2ActivityTracker,
     /// The unrecorded blocks that are used to calculate the projected cost of recording blocks
     pub unrecorded_blocks: BTreeMap<Height, Bytes>,
 }
 
-/// A value that represents a value between 0 and 100. Higher values are clamped to 100
+/// The `L2ActivityTracker` tracks the chain activity to determine a safety mode for setting the DA price.
+///
+/// Because the DA gas price can increase even when no-one is using the network, there is a potential
+/// for a negative feedback loop to occur where the gas price increases, further decreasing activity
+/// and increasing the gas price. The `L2ActivityTracker` is used to moderate changes to the DA
+/// gas price based on the activity of the L2 chain.
+///
+/// The chain activity is a cumulative measure, updated whenever a new block is processed.
+/// For each L2 block, the block usage is a percentage of the block capacity used. If the
+/// block usage is below a certain threshold, the chain activity is decreased, if above the threshold,
+/// the activity is increased The chain activity exists on a scale
+/// between 0 and the sum of the normal, hold, and decrease buffers.
+///
+/// e.g. if the decrease buffer size is 20, the hold buffer size is 60, and the increase buffer size is 40:
+///
+/// 0<-- decrease buffer -->20<-- hold buffer -->80<-- normal buffer -->120
+///
+/// The current chain activity determines the behavior of the DA gas price.
+///
+/// For healthy behavior, the activity should be in the `normal` range.
 #[derive(serde::Serialize, serde::Deserialize, Debug, Clone, PartialEq)]
+pub struct L2ActivityTracker {
+    /// The maximum value the chain activity can hit
+    max_activity: u16,
+    /// The threshold if the block activity is below, the DA gas price will be held when it would otherwise be increased
+    hold_activity_threshold: u16,
+    /// If the chain activity falls below this value, the DA gas price will be decreased when it would otherwise be increased
+    decrease_activity_threshold: u16,
+    /// The current activity of the L2 chain
+    chain_activity: u16,
+    /// The threshold of block activity below which the chain activity will be decreased,
+    /// above or equal it will always increase
+    block_activity_threshold: ClampedPercentage,
+}
+
+/// Designates the intended behavior of the DA gas price based on the activity of the L2 chain
+pub enum DAGasPriceSafetyMode {
+    /// Should increase DA gas price freely
+    Normal,
+    /// Should not increase the DA gas price
+    Hold,
+    /// Should decrease the DA gas price always
+    Decrease,
+}
+
+impl L2ActivityTracker {
+    pub fn new_full(
+        normal_range_size: u16,
+        hold_range_size: u16,
+        decrease_range_size: u16,
+        block_activity_threshold: ClampedPercentage,
+    ) -> Self {
+        let decrease_activity_threshold = decrease_range_size;
+        let hold_activity_threshold = decrease_range_size.saturating_add(hold_range_size);
+        let max_activity = hold_activity_threshold.saturating_add(normal_range_size);
+        let chain_activity = max_activity;
+        Self {
+            max_activity,
+            hold_activity_threshold,
+            decrease_activity_threshold,
+            chain_activity,
+            block_activity_threshold,
+        }
+    }
+
+    pub fn new(
+        normal_range_size: u16,
+        hold_range_size: u16,
+        decrease_range_size: u16,
+        activity: u16,
+        block_activity_threshold: ClampedPercentage,
+    ) -> Self {
+        let mut tracker = Self::new_full(
+            normal_range_size,
+            hold_range_size,
+            decrease_range_size,
+            block_activity_threshold,
+        );
+        tracker.chain_activity = activity.min(tracker.max_activity);
+        tracker
+    }
+
+    pub fn new_always_normal() -> Self {
+        let normal_range_size = 100;
+        let hold_range_size = 0;
+        let decrease_range_size = 0;
+        let percentage = ClampedPercentage::new(0);
+        Self::new(
+            normal_range_size,
+            hold_range_size,
+            decrease_range_size,
+            100,
+            percentage,
+        )
+    }
+
+    pub fn safety_mode(&self) -> DAGasPriceSafetyMode {
+        if self.chain_activity > self.hold_activity_threshold {
+            DAGasPriceSafetyMode::Normal
+        } else if self.chain_activity > self.decrease_activity_threshold {
+            DAGasPriceSafetyMode::Hold
+        } else {
+            DAGasPriceSafetyMode::Decrease
+        }
+    }
+
+    pub fn update(&mut self, block_usage: ClampedPercentage) {
+        if block_usage < self.block_activity_threshold {
+            self.chain_activity = self.chain_activity.saturating_sub(1);
+        } else {
+            self.chain_activity =
+                self.chain_activity.saturating_add(1).min(self.max_activity);
+        }
+    }
+
+    pub fn current_activity(&self) -> u16 {
+        self.chain_activity
+    }
+}
+
+/// A value that represents a value between 0 and 100. Higher values are clamped to 100
+#[derive(serde::Serialize, serde::Deserialize, Debug, Clone, PartialEq, PartialOrd)]
 pub struct ClampedPercentage {
     value: u8,
 }
@@ -226,6 +342,9 @@ impl AlgorithmUpdaterV1 {
             let last_profit = rewards.saturating_sub(projected_total_da_cost);
             self.update_last_profit(last_profit);
 
+            // activity
+            self.update_activity(used, capacity);
+
             // gas prices
             self.update_exec_gas_price(used, capacity);
             self.update_da_gas_price();
@@ -236,6 +355,12 @@ impl AlgorithmUpdaterV1 {
         }
     }
 
+    fn update_activity(&mut self, used: u64, capacity: NonZeroU64) {
+        let block_activity = used.saturating_mul(100).div(capacity);
+        let usage = ClampedPercentage::new(block_activity.try_into().unwrap_or(100));
+        self.l2_activity.update(usage);
+    }
+
     fn update_da_rewards(&mut self, fee_wei: u128) {
         let block_da_reward = self.da_portion_of_fee(fee_wei);
         self.total_da_rewards_excess =
@@ -309,7 +434,8 @@ impl AlgorithmUpdaterV1 {
     fn update_da_gas_price(&mut self) {
         let p = self.p();
         let d = self.d();
-        let da_change = self.da_change(p, d);
+        let maybe_da_change = self.da_change(p, d);
+        let da_change = self.da_change_accounting_for_activity(maybe_da_change);
         let maybe_new_scaled_da_gas_price = i128::from(self.new_scaled_da_gas_price)
             .checked_add(da_change)
             .and_then(|x| u64::try_from(x).ok())
@@ -326,6 +452,18 @@ impl AlgorithmUpdaterV1 {
         );
     }
 
+    fn da_change_accounting_for_activity(&self, maybe_da_change: i128) -> i128 {
+        if maybe_da_change > 0 {
+            match self.l2_activity.safety_mode() {
+                DAGasPriceSafetyMode::Normal => maybe_da_change,
+                DAGasPriceSafetyMode::Hold => 0,
+                DAGasPriceSafetyMode::Decrease => self.max_change().saturating_mul(-1),
+            }
+        } else {
+            maybe_da_change
+        }
+    }
+
     fn min_scaled_da_gas_price(&self) -> u64 {
         self.min_da_gas_price
             .saturating_mul(self.gas_price_factor.into())
@@ -348,14 +486,18 @@ impl AlgorithmUpdaterV1 {
 
     fn da_change(&self, p: i128, d: i128) -> i128 {
         let pd_change = p.saturating_add(d);
+        let max_change = self.max_change();
+        let clamped_change = pd_change.saturating_abs().min(max_change);
+        pd_change.signum().saturating_mul(clamped_change)
+    }
+
+    // Should always be positive
+    fn max_change(&self) -> i128 {
         let upcast_percent = self.max_da_gas_price_change_percent.into();
-        let max_change = self
-            .new_scaled_da_gas_price
+        self.new_scaled_da_gas_price
             .saturating_mul(upcast_percent)
             .saturating_div(100)
-            .into();
-        let clamped_change = pd_change.saturating_abs().min(max_change);
-        pd_change.signum().saturating_mul(clamped_change)
+            .into()
     }
 
     fn exec_change(&self, principle: u64) -> u64 {

crates/fuel-gas-price-algorithm/src/v1/tests.rs

@@ -2,7 +2,10 @@
 #![allow(clippy::arithmetic_side_effects)]
 #![allow(clippy::cast_possible_truncation)]
 
-use crate::v1::AlgorithmUpdaterV1;
+use crate::v1::{
+    AlgorithmUpdaterV1,
+    L2ActivityTracker,
+};
 
 #[cfg(test)]
 mod algorithm_v1_tests;
@@ -40,6 +43,7 @@ pub struct UpdaterBuilder {
     last_profit: i128,
     second_to_last_profit: i128,
     da_gas_price_factor: u64,
+    l2_activity: L2ActivityTracker,
 }
 
 impl UpdaterBuilder {
@@ -67,6 +71,7 @@ impl UpdaterBuilder {
             last_profit: 0,
             second_to_last_profit: 0,
             da_gas_price_factor: 1,
+            l2_activity: L2ActivityTracker::new_always_normal(),
         }
     }
 
@@ -159,6 +164,11 @@ impl UpdaterBuilder {
         self
     }
 
+    fn with_activity(mut self, l2_activity: L2ActivityTracker) -> Self {
+        self.l2_activity = l2_activity;
+        self
+    }
+
     fn build(self) -> AlgorithmUpdaterV1 {
         AlgorithmUpdaterV1 {
             min_exec_gas_price: self.min_exec_gas_price,
@@ -190,6 +200,7 @@ impl UpdaterBuilder {
                 .da_gas_price_factor
                 .try_into()
                 .expect("Should never be non-zero"),
+            l2_activity: self.l2_activity,
         }
     }
 }