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jthomson04 merged 2 commits into from
Sep 28, 2026
Merged

perf(sglang): reduce admission checkpoint copies #321

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0edc5c2
perf(sglang): reduce admission checkpoint copies
jthomson04 Sep 23, 2026
294e3a1
perf(sglang): reduce remaining admission copies and test rollback
jthomson04 Sep 23, 2026
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129 changes: 94 additions & 35 deletions crates/core/src/engine/cache/radix_cache.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ use rustc_hash::FxHashMap;
use slotmap::{SlotMap, new_key_type};
use std::cmp::Reverse;
use std::collections::BTreeSet;
use std::sync::Arc;
use std::time::Instant;

new_key_type! {
Expand Down Expand Up @@ -169,9 +170,11 @@ pub struct TreeNode {
/// page identity so completed radix state does not retain token IDs.
/// Consequently, as in router-side indexing, hash collisions are treated
/// as identical pages rather than guarded by an exact-token comparison.
pub key: Vec<LocalBlockHash>,
/// Admission snapshots share edge contents. Copy only an edge that changes
/// while a snapshot is live; normal decode extensions retain their capacity.
pub key: Arc<Vec<LocalBlockHash>>,
/// One physical page ID per key. Length = `key.len()`.
pub value: Vec<KvPageId>,
pub value: Arc<Vec<KvPageId>>,
/// Walk-to-root reference count (protected when > 0).
pub lock_ref: usize,
/// Monotonic timestamp for LRU eviction.
Expand Down Expand Up @@ -207,6 +210,30 @@ struct BeladyLeaves {
indexed: FxHashMap<NodeId, (SequenceHash, usize)>,
}

/// Leave the suffix in `edge` and return a compact prefix. When a snapshot
/// shares the edge, copy each half once instead of cloning before splitting.
fn split_edge_prefix<T: Clone>(edge: &mut Arc<Vec<T>>, at: usize) -> Vec<T> {
if let Some(values) = Arc::get_mut(edge) {
let suffix = values.split_off(at);
let mut prefix = std::mem::replace(values, suffix);
// The short prefix must not retain the original edge's allocation.
prefix.shrink_to_fit();
prefix
} else {
let prefix = edge[..at].to_vec();
*edge = Arc::new(edge[at..].to_vec());
prefix
}
}

fn truncate_edge<T: Clone>(edge: &mut Arc<Vec<T>>, len: usize) {
if let Some(values) = Arc::get_mut(edge) {
values.truncate(len);
} else {
*edge = Arc::new(edge[..len].to_vec());
}
}

impl RadixCache {
/// Snapshot allocator and radix metadata for a fallible admission transaction.
/// This is not a second owner of live request leases; only one cache state may commit.
Expand All @@ -231,8 +258,8 @@ impl RadixCache {
let root = nodes.insert(TreeNode {
children: FxHashMap::default(),
parent: None,
key: Vec::new(),
value: Vec::new(),
key: Arc::default(),
value: Arc::default(),
lock_ref: 0,
last_access_time: Instant::now(),
});
Expand Down Expand Up @@ -810,8 +837,8 @@ impl RadixCache {
node.last_access_time = now;
debug_assert!(node.children.is_empty());
debug_assert!(node.lock_ref <= 1);
node.key.extend_from_slice(key);
node.value.extend_from_slice(value);
Arc::make_mut(&mut node.key).extend_from_slice(key);
Arc::make_mut(&mut node.value).extend_from_slice(value);
if node.lock_ref == 0 {
self.evictable_size += key.len() * self.page_size;
} else {
Expand All @@ -830,13 +857,8 @@ impl RadixCache {
let child = &mut self.nodes[child_id];
let child_parent = child.parent;
let original_ck = child.key[0];
let suffix_key = child.key.split_off(split_pos);
let mut prefix_key = std::mem::replace(&mut child.key, suffix_key);
let suffix_value = child.value.split_off(split_pos);
let mut prefix_value = std::mem::replace(&mut child.value, suffix_value);
// The short prefix must not retain the original edge's allocation.
prefix_key.shrink_to_fit();
prefix_value.shrink_to_fit();
let prefix_key = split_edge_prefix(&mut child.key, split_pos);
let prefix_value = split_edge_prefix(&mut child.value, split_pos);
let suffix_ck = child.key[0];
(
child_parent,
Expand All @@ -855,8 +877,8 @@ impl RadixCache {
let intermediate = TreeNode {
children: inter_children,
parent: child_parent,
key: prefix_key,
value: prefix_value,
key: Arc::new(prefix_key),
value: Arc::new(prefix_value),
lock_ref,
last_access_time: accessed,
};
Expand Down Expand Up @@ -886,8 +908,8 @@ impl RadixCache {
let new_node = TreeNode {
children: FxHashMap::default(),
parent: Some(parent_id),
key: key.to_vec(),
value: value.to_vec(),
key: Arc::new(key.to_vec()),
value: Arc::new(value.to_vec()),
lock_ref: 0,
last_access_time: now,
};
Expand Down Expand Up @@ -1015,7 +1037,7 @@ impl RadixCache {
let split_pos = victim_pages - eviction_pages;
let (nodes, page_pool) = (&mut self.nodes, &mut self.page_pool);
let victim_node = &mut nodes[victim_id];
victim_node.key.truncate(split_pos);
truncate_edge(&mut victim_node.key, split_pos);
let evicted_values = &victim_node.value[split_pos..];
if let Some(belady) = &mut self.belady {
for page in evicted_values {
Expand All @@ -1024,7 +1046,7 @@ impl RadixCache {
}
page_pool.free_pages(evicted_values);
evicted_indices.extend_from_slice(evicted_values);
victim_node.value.truncate(split_pos);
truncate_edge(&mut victim_node.value, split_pos);

self.evictable_size -= eviction_len;
evicted += eviction_len;
Expand All @@ -1044,7 +1066,7 @@ impl RadixCache {

evicted_indices.extend_from_slice(&victim_node.value);
if let Some(belady) = &mut self.belady {
for page in &victim_node.value {
for page in victim_node.value.iter() {
belady.page_hashes[page.index()] = None;
}
}
Expand Down Expand Up @@ -1097,20 +1119,29 @@ mod tests {
.unwrap()
}

#[test]
fn belady_reranks_compressed_tail_before_evicting_its_prefix() {
#[rstest::rstest]
fn belady_reranks_compressed_tail_before_evicting_its_prefix(
#[values(false, true)] shared: bool,
) {
let mut cache = RadixCache::new(3, 1);
cache.set_belady_oracle(oracle_for_prompts(&[&[1], &[3], &[1, 2]]));
let ab = cache.page_pool.allocate(2).unwrap();
cache.insert(&[1, 2], &ab);
let c = cache.page_pool.allocate(1).unwrap();
cache.insert(&[3], &c);
let snapshot = shared.then(|| cache.admission_checkpoint());

let (tokens, pages) = cache.evict(2);
assert_eq!((tokens, pages), (2, vec![KvPageId(1), KvPageId(2)]));
assert_eq!(cache.prefix_match_len(&[1, 2]), 1);
assert_eq!(cache.prefix_match_len(&[3]), 0);
assert_eq!(cache.available_tokens(), 2);
if let Some(mut saved) = snapshot {
assert_eq!(saved.prefix_match_len(&[1, 2]), 2);
assert_eq!(saved.prefix_match_len(&[3]), 1);
assert_eq!(saved.available_tokens(), 0);
assert_eq!(saved.evict(2).1, vec![KvPageId(1), KvPageId(2)]);
}
}

#[test]
Expand Down Expand Up @@ -1239,9 +1270,9 @@ mod tests {
let (len, node) = cache.match_prefix(&[1, 2, 3, 4, 5, 9, 9]);
assert_eq!(len, 5);
let n = cache.node(node);
assert_eq!(n.key, cache.page_hashes(&[1, 2, 3, 4, 5]));
assert_eq!(*n.key, cache.page_hashes(&[1, 2, 3, 4, 5]));
assert_eq!(
n.value,
*n.value,
vec![
KvPageId(10),
KvPageId(20),
Expand All @@ -1253,7 +1284,7 @@ mod tests {
let suffix_key = cache.page_hashes(&[6])[0];
let &suffix_id = n.children.get(&suffix_key).unwrap();
assert_eq!(
cache.node(suffix_id).value,
*cache.node(suffix_id).value,
vec![KvPageId(60), KvPageId(70)]
);
}
Expand Down Expand Up @@ -1286,15 +1317,31 @@ mod tests {
assert_eq!(cache.match_prefix(&[1, 2, 3, 4, 5, 6, 7, 8]).0, 8);
}

#[test]
fn repeated_prefix_splits_keep_retained_capacity_proportional_to_live_pages() {
#[rstest::rstest]
fn repeated_prefix_splits_keep_retained_capacity_proportional_to_live_pages(
#[values(false, true)] shared: bool,
) {
let mut cache = RadixCache::new(512, 1);
let tokens: Vec<u32> = (0..512).collect();
let indices: Vec<usize> = (0..512).collect();
cache.insert(&tokens, &indices);

for prefix_len in (1..512).step_by(4) {
let snapshot = shared.then(|| cache.admission_checkpoint());
let saved_edges = snapshot.as_ref().map(|saved| {
saved
.nodes
.iter()
.map(|(id, node)| (id, (node.key.to_vec(), node.value.to_vec())))
.collect::<FxHashMap<_, _>>()
});
assert_eq!(cache.match_prefix(&tokens[..prefix_len]).0, prefix_len);
if let Some(saved) = snapshot {
for (id, (key, pages)) in saved_edges.unwrap() {
assert_eq!(*saved.node(id).key, key);
assert_eq!(*saved.node(id).value, pages);
}
}
}
assert_eq!(cache.match_prefix(&tokens).0, tokens.len());

Expand Down Expand Up @@ -1331,7 +1378,7 @@ mod tests {
assert_eq!(extended, tail);
assert_eq!(cache.num_nodes(), nodes_before);
assert_eq!(
cache.node(tail).key,
*cache.node(tail).key,
cache.page_hashes(&[1, 2, 3, 4, 5, 6, 7, 8])
);
assert_eq!(cache.protected_size, 8);
Expand All @@ -1356,8 +1403,8 @@ mod tests {

assert_ne!(extended, tail);
assert_eq!(cache.num_nodes(), nodes_before + 1);
assert_eq!(cache.node(tail).key, cache.page_hashes(&[1, 2, 3, 4]));
assert_eq!(cache.node(extended).key, cache.page_hashes(&[5, 6, 7, 8]));
assert_eq!(*cache.node(tail).key, cache.page_hashes(&[1, 2, 3, 4]));
assert_eq!(*cache.node(extended).key, cache.page_hashes(&[5, 6, 7, 8]));
}

#[test]
Expand All @@ -1368,7 +1415,7 @@ mod tests {
cache.insert(&[1, 2, 3, 4, 5, 6, 7], &[0, 1, 2, 3, 4, 5, 6]);
assert_eq!(cache.match_prefix(&[1, 2, 3, 4]).0, 4);
let (_, node) = cache.match_prefix(&[1, 2, 3, 4]);
assert_eq!(cache.node(node).value, vec![KvPageId(0)]);
assert_eq!(*cache.node(node).value, vec![KvPageId(0)]);

cache.insert(&[1, 2, 3, 4, 5, 6, 7, 8], &[0, 1, 2, 3, 4, 5, 6, 7]);
assert_eq!(cache.match_prefix(&[1, 2, 3, 4, 5, 6, 7, 8]).0, 8);
Expand All @@ -1386,7 +1433,7 @@ mod tests {
cache.insert(&[1, 2, 3, 4, 5, 6, 7, 8], &[0, 1, 2, 3, 4, 5, 6, 7]);
cache.match_prefix(&[1, 2, 3, 4, 9, 9, 9, 9]);
let (_, node) = cache.match_prefix(&[1, 2, 3, 4]);
assert_eq!(cache.node(node).value, vec![KvPageId(0)]);
assert_eq!(*cache.node(node).value, vec![KvPageId(0)]);
}

#[test]
Expand All @@ -1398,7 +1445,7 @@ mod tests {

assert_eq!(cache.node(node).key.len(), 2);
assert_eq!(cache.node(node).value.len(), 2);
assert_eq!(cache.node(node).value, vec![KvPageId(0), KvPageId(1)]);
assert_eq!(*cache.node(node).value, vec![KvPageId(0), KvPageId(1)]);
assert_eq!(cache.match_prefix(&tokens).0, tokens.len());
}

Expand Down Expand Up @@ -1511,22 +1558,34 @@ mod tests {
assert_eq!(cache.match_prefix(&[4, 5, 6]).0, 3);
}

#[test]
fn partial_eviction_and_parent_promotion_keep_index_consistent() {
#[rstest::rstest]
fn partial_eviction_and_parent_promotion_keep_index_consistent(
#[values(false, true)] shared: bool,
) {
let mut cache = RadixCache::new(100, 1);
let base = Instant::now();
cache.set_test_now(base);
cache.insert(&[1, 2, 3, 4], &[0, 1, 2, 3]);
let (_, leaf) = cache.match_prefix(&[1, 2, 3, 4]);
cache.set_test_now(base + std::time::Duration::from_secs(1));
cache.insert(&[5, 6, 7], &[4, 5, 6]);
let snapshot = shared.then(|| cache.admission_checkpoint());

assert_eq!(cache.evict(2).0, 2);
assert_eq!(cache.node(leaf).key.len(), 2);
assert!(cache.evictable_leaf_is_indexed(leaf));
assert_eq!(cache.evict(2).0, 2);
assert_eq!(cache.match_prefix(&[1, 2, 3, 4]).0, 0);
assert_eq!(cache.match_prefix(&[5, 6, 7]).0, 3);
if let Some(mut saved) = snapshot {
assert_eq!(*saved.node(leaf).key, saved.page_hashes(&[1, 2, 3, 4]));
assert_eq!(
*saved.node(leaf).value,
vec![KvPageId(0), KvPageId(1), KvPageId(2), KvPageId(3)]
);
assert!(saved.evictable_leaf_is_indexed(leaf));
assert_eq!(saved.evict(2).1, vec![KvPageId(2), KvPageId(3)]);
}

let mut branched = RadixCache::new(100, 1);
branched.set_test_now(base);
Expand Down
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