A0-4277: Unit creation delay (#1707)

# Description

Change `DEFAULT_UNIT_CREATION_DELAY` from `300` ms to `200` ms. Change
`MAX_ROUNDS` and exponential slowdown params accordingly so that the
time to reach it after updating unit creation delay is similar to the
time before the update.

Change the starting round of exponential slowdown from proportionally,
i.e. from 5000 rounds to 7500 rounds, so that the no-slowdown time is 25
min which remains significantly longer than the expected session time
(15 min).

## Testing
The following dummy test-case in AlephBFT:
```rust
let t1: Duration = (0..7000).map(|t| exponential_slowdown(t, 300.0, 5000, 1.005)).sum();
let t2: Duration = (0..10000).map(|t| exponential_slowdown(t, 200.0, 7500, 1.004)).sum();
println!("T1: {:?}", t1);
println!("T2: {:?}", t2);
```
gave output:
```
T1: 1290504.822s
T2: 1081023.038s
```
which is ~14.9 and ~12.5 days correspondingly. This means that the new
time to reach max round will be 12.5 days which is not much smaller than
14.9 days, and at the same time, noticeably longer than the required 7
days.

**Reference**: [exponential
slowdown](https://github.com/Cardinal-Cryptography/AlephBFT/blob/main/consensus/src/config.rs#L97)

## Type of change
- New feature (non-breaking change which adds functionality)

docker/common.yml

@@ -8,7 +8,7 @@ services:
       - CHAIN=/data/chainspec.json
       - ALLOW_PRIVATE_IPV4=true
       - DISCOVER_LOCAL=true
-      - UNIT_CREATION_DELAY=300
+      - UNIT_CREATION_DELAY=200
     volumes:
       - ./data/:/data/
 

docker/docker_entrypoint.sh

@@ -28,7 +28,7 @@ PROMETHEUS_ENABLED=${PROMETHEUS_ENABLED:-true}
 TELEMETRY_ENABLED=${TELEMETRY_ENABLED:-false}
 TELEMETRY_URL=${TELEMETRY_URL:-'wss://telemetry.polkadot.io/submit/'}
 TELEMETRY_VERBOSITY_LVL=${TELEMETRY_VERBOSITY_LVL:-'0'}
-UNIT_CREATION_DELAY=${UNIT_CREATION_DELAY:-300}
+UNIT_CREATION_DELAY=${UNIT_CREATION_DELAY:-200}
 DB_CACHE=${DB_CACHE:-1024}
 RUNTIME_CACHE_SIZE=${RUNTIME_CACHE_SIZE:-2}
 MAX_RUNTIME_INSTANCES=${MAX_RUNTIME_INSTANCES:-8}

docker/smartnet-compose.yml

@@ -16,7 +16,7 @@ services:
       - PURGE_BEFORE_START=false
       - RPC_PORT=9943
       - RUST_LOG=info
-      - UNIT_CREATION_DELAY=300
+      - UNIT_CREATION_DELAY=200
       - BOOT_NODES=/ip4/127.0.0.1/tcp/30333/p2p/$BOOTNODE_PEER_ID
       - PUBLIC_ADDR=/ip4/127.0.0.1/tcp/30333
       - VALIDATOR_PORT=30343

finality-aleph/src/abft/common.rs

@@ -2,7 +2,15 @@ use std::{sync::Arc, time::Duration};
 
 use crate::UnitCreationDelay;
 
-pub const MAX_ROUNDS: u16 = 7000;
+// Chosen as a round number large enough so that given the default 200 ms unit creation delay, and the exponential
+// slowdown consts below, the time to reach the max round noticeably surpasses the required 7 days. With this
+// setting max round should be reached in ~12.5 days.
+pub const MAX_ROUNDS: u16 = 10_000;
+
+// Given the default 200 ms unit creation delay, the expected no-slowdown time will be 7500*200/1000/60 = 25 minutes
+// which is noticeably longer than the expected 15 minutes of session time.
+const EXP_SLOWDOWN_START_ROUND: usize = 7500;
+const EXP_SLOWDOWN_MUL: f64 = 1.004;
 
 fn exponential_slowdown(
     t: usize,
@@ -29,7 +37,12 @@ pub type DelaySchedule = Arc<dyn Fn(usize) -> Duration + Sync + Send + 'static>;
 pub fn unit_creation_delay_fn(unit_creation_delay: UnitCreationDelay) -> DelaySchedule {
     Arc::new(move |t| match t {
         0 => Duration::from_millis(2000),
-        _ => exponential_slowdown(t, unit_creation_delay.0 as f64, 5000, 1.005),
+        _ => exponential_slowdown(
+            t,
+            unit_creation_delay.0 as f64,
+            EXP_SLOWDOWN_START_ROUND,
+            EXP_SLOWDOWN_MUL,
+        ),
     })
 }
 

primitives/src/lib.rs

@@ -136,7 +136,7 @@ pub const TOKEN: u128 = 10u128.pow(TOKEN_DECIMALS);
 pub const ADDRESSES_ENCODING: u8 = 42;
 
 /// ABFT unit creation delay (in ms)
-pub const DEFAULT_UNIT_CREATION_DELAY: u64 = 300;
+pub const DEFAULT_UNIT_CREATION_DELAY: u64 = 200;
 
 /// Committee Size for new chains
 pub const DEFAULT_COMMITTEE_SIZE: u32 = 4;