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chore(pageserver): improve in-memory layer vectored get (#7467)

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previously in https://github.com/neondatabase/neon/pull/7375, we
observed that for in-memory layers, we will need to iterate every key in
the key space in order to get the result. The operation can be more
efficient if we use BTreeMap as the in-memory layer representation, even
if we are doing vectored get in a dense keyspace. Imagine a case that
the in-memory layer covers a very little part of the keyspace, and most
of the keys need to be found in lower layers. Using a BTreeMap can
significantly reduce probes for nonexistent keys.

## Summary of changes

* Use BTreeMap as in-memory layer representation.
* Optimize the vectored get flow to utilize the range scan functionality
of BTreeMap.

Signed-off-by: Alex Chi Z <chi@neon.tech>
This commit is contained in:
Alex Chi Z
2024-04-29 13:16:42 -04:00
committed by GitHub
parent cddafc79e1
commit 11945e64ec
1 changed files with 18 additions and 32 deletions
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50
pageserver/src/tenant/storage_layer/inmemory_layer.rs
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@@ -17,7 +17,7 @@ use anyhow::{anyhow, ensure, Result};
use pageserver_api::keyspace::KeySpace;
use pageserver_api::models::InMemoryLayerInfo;
use pageserver_api::shard::TenantShardId;
use std::collections::{BinaryHeap, HashMap, HashSet};
use std::collections::{BTreeMap, BinaryHeap, HashSet};
use std::sync::{Arc, OnceLock};
use std::time::Instant;
use tracing::*;
@@ -78,10 +78,10 @@ impl std::fmt::Debug for InMemoryLayer {
}
pub struct InMemoryLayerInner {
/// All versions of all pages in the layer are kept here. Indexed
/// All versions of all pages in the layer are kept here. Indexed
/// by block number and LSN. The value is an offset into the
/// ephemeral file where the page version is stored.
index: HashMap<Key, VecMap<Lsn, u64>>,
index: BTreeMap<Key, VecMap<Lsn, u64>>,
/// The values are stored in a serialized format in this file.
/// Each serialized Value is preceded by a 'u32' length field.
@@ -384,25 +384,20 @@ impl InMemoryLayer {
let mut planned_block_reads = BinaryHeap::new();
for range in keyspace.ranges.iter() {
let mut key = range.start;
while key < range.end {
if let Some(vec_map) = inner.index.get(&key) {
let lsn_range = match reconstruct_state.get_cached_lsn(&key) {
Some(cached_lsn) => (cached_lsn + 1)..end_lsn,
None => self.start_lsn..end_lsn,
};
for (key, vec_map) in inner.index.range(range.start..range.end) {
let lsn_range = match reconstruct_state.get_cached_lsn(key) {
Some(cached_lsn) => (cached_lsn + 1)..end_lsn,
None => self.start_lsn..end_lsn,
};
let slice = vec_map.slice_range(lsn_range);
for (entry_lsn, pos) in slice.iter().rev() {
planned_block_reads.push(BlockRead {
key,
lsn: *entry_lsn,
block_offset: *pos,
});
}
let slice = vec_map.slice_range(lsn_range);
for (entry_lsn, pos) in slice.iter().rev() {
planned_block_reads.push(BlockRead {
key: *key,
lsn: *entry_lsn,
block_offset: *pos,
});
}
key = key.next();
}
}
@@ -499,7 +494,7 @@ impl InMemoryLayer {
end_lsn: OnceLock::new(),
opened_at: Instant::now(),
inner: RwLock::new(InMemoryLayerInner {
index: HashMap::new(),
index: BTreeMap::new(),
file,
resource_units: GlobalResourceUnits::new(),
}),
@@ -636,26 +631,17 @@ impl InMemoryLayer {
let cursor = inner.file.block_cursor();
// Sort the keys because delta layer writer expects them sorted.
//
// NOTE: this sort can take up significant time if the layer has millions of
// keys. To speed up all the comparisons we convert the key to i128 and
// keep the value as a reference.
let mut keys: Vec<_> = inner.index.iter().map(|(k, m)| (k.to_i128(), m)).collect();
keys.sort_unstable_by_key(|k| k.0);
let ctx = RequestContextBuilder::extend(ctx)
.page_content_kind(PageContentKind::InMemoryLayer)
.build();
for (key, vec_map) in keys.iter() {
let key = Key::from_i128(*key);
for (key, vec_map) in inner.index.iter() {
// Write all page versions
for (lsn, pos) in vec_map.as_slice() {
cursor.read_blob_into_buf(*pos, &mut buf, &ctx).await?;
let will_init = Value::des(&buf)?.will_init();
let res;
(buf, res) = delta_layer_writer
.put_value_bytes(key, *lsn, buf, will_init)
.put_value_bytes(*key, *lsn, buf, will_init)
.await;
res?;
}