improve shard_amount usage

sokra · sokra · commit 0933cf94d49f · 2025-10-16T20:44:30.000+02:00
diff --git a/turbopack/crates/turbo-tasks-backend/src/backend/mod.rs b/turbopack/crates/turbo-tasks-backend/src/backend/mod.rs
@@ -15,7 +15,6 @@ use std::{
         Arc,
         atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering},
     },
-    thread::available_parallelism,
 };
 
 use anyhow::{Result, bail};
@@ -67,7 +66,7 @@ use crate::{
     },
     utils::{
         bi_map::BiMap, chunked_vec::ChunkedVec, dash_map_drop_contents::drop_contents,
-        ptr_eq_arc::PtrEqArc, sharded::Sharded, swap_retain,
+        ptr_eq_arc::PtrEqArc, shard_amount::compute_shard_amount, sharded::Sharded, swap_retain,
     },
 };
 
@@ -233,14 +232,12 @@ impl<B: BackingStorage> TurboTasksBackend<B> {
 
 impl<B: BackingStorage> TurboTasksBackendInner<B> {
     pub fn new(mut options: BackendOptions, backing_storage: B) -> Self {
-        let shard_amount =
-            (available_parallelism().map_or(4, |v| v.get()) * 64).next_power_of_two();
+        let shard_amount = compute_shard_amount(options.num_workers, options.small_preallocation);
         let need_log = matches!(options.storage_mode, Some(StorageMode::ReadWrite));
         if !options.dependency_tracking {
             options.active_tracking = false;
         }
         let small_preallocation = options.small_preallocation;
-        let num_workers = options.num_workers;
         let next_task_id = backing_storage
             .next_free_task_id()
             .expect("Failed to get task id");
@@ -262,7 +259,7 @@ impl<B: BackingStorage> TurboTasksBackendInner<B> {
             task_cache: BiMap::new(),
             transient_tasks: FxDashMap::default(),
             local_is_partial: AtomicBool::new(next_task_id != TaskId::MIN),
-            storage: Storage::new(num_workers, small_preallocation),
+            storage: Storage::new(shard_amount, small_preallocation),
             in_progress_operations: AtomicUsize::new(0),
             snapshot_request: Mutex::new(SnapshotRequest::new()),
             operations_suspended: Condvar::new(),
diff --git a/turbopack/crates/turbo-tasks-backend/src/backend/storage.rs b/turbopack/crates/turbo-tasks-backend/src/backend/storage.rs
@@ -2,7 +2,6 @@ use std::{
     hash::Hash,
     ops::{Deref, DerefMut},
     sync::{Arc, atomic::AtomicBool},
-    thread::available_parallelism,
 };
 
 use bitfield::bitfield;
@@ -616,19 +615,13 @@ pub struct Storage {
 }
 
 impl Storage {
-    pub fn new(num_worker: Option<usize>, small_preallocation: bool) -> Self {
+    pub fn new(shard_amount: usize, small_preallocation: bool) -> Self {
         let map_capacity: usize = if small_preallocation {
             1024
         } else {
             1024 * 1024
         };
         let modified_capacity: usize = if small_preallocation { 0 } else { 1024 };
-        let shard_factor: usize = if small_preallocation { 1 } else { 16 };
-
-        let num_workers =
-            num_worker.unwrap_or_else(|| available_parallelism().map_or(4, |v| v.get()));
-
-        let shard_amount = (num_workers * num_workers * shard_factor).next_power_of_two();
 
         Self {
             snapshot_mode: AtomicBool::new(false),
diff --git a/turbopack/crates/turbo-tasks-backend/src/utils/mod.rs b/turbopack/crates/turbo-tasks-backend/src/utils/mod.rs
@@ -3,6 +3,7 @@ pub mod chunked_vec;
 pub mod dash_map_drop_contents;
 pub mod dash_map_multi;
 pub mod ptr_eq_arc;
+pub mod shard_amount;
 pub mod sharded;
 pub mod swap_retain;
 
diff --git a/turbopack/crates/turbo-tasks-backend/src/utils/shard_amount.rs b/turbopack/crates/turbo-tasks-backend/src/utils/shard_amount.rs
@@ -0,0 +1,44 @@
+use std::thread::available_parallelism;
+
+/// Compute a good number of shards to use for sharded data structures.
+/// The number of shards is computed based on the number of worker threads
+/// and whether a small preallocation is requested.
+/// A small preallocation is useful for tests where performance is not
+/// critical and we want to reduce memory usage and startup time.
+/// The number of shards is chosen to minimize the probability of shard
+/// collisions (which can lead to false sharing) while keeping memory
+/// usage reasonable.
+/// The returned number is always a power of two as this is often required
+/// by sharded data structures. The maximum shard amount is capped at 1 << 16 (65536).
+pub fn compute_shard_amount(num_workers: Option<usize>, small_preallocation: bool) -> usize {
+    let num_workers = num_workers.unwrap_or_else(|| available_parallelism().map_or(4, |v| v.get()));
+
+    // Once can compute the probability of a shard collision (which leads to false sharing) using
+    // the birthday paradox formula.  It's notable that the probability of collisions increases
+    // with more worker threads. To mitigate this effect, the number of shards need to grow
+    // quadratically with the number of worker threads. This way the probability of at least one
+    // collision remains constant.
+    //
+    // Lets call the worker thread count `N` and the number of shards `S`. When using `S = k * N^2`
+    // for some constant `k` the probability of at least one collision for large `N` can be
+    // approximated as: P = 1 - exp(-N^2 / (2*S)) = 1 - exp(-1/(2*k))
+    //
+    // For `k = 16` this results in a collision probability of about 3%.
+    // For `k = 1` this results in a collision probability of about 39%.
+    //
+    // We clamp the number of shards to 1 << 16 to avoid excessive memory usage in case of a very
+    // high number of worker threads. This case is hit with more than 64 worker threads for `k =
+    // 16` and more than 256 worker threads for `k = 1`.
+
+    if small_preallocation {
+        // We also clamp the minimum number of workers to 256 so all following multiplications can't
+        // overflow.
+        let num_workers = num_workers.max(256);
+        (num_workers * num_workers).next_power_of_two()
+    } else {
+        // We also clamp the minimum number of workers to 64 so all following multiplications can't
+        // overflow.
+        let num_workers = num_workers.max(64);
+        (num_workers * num_workers * 16).next_power_of_two()
+    }
+}
