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pageserver: skip unnecessary fringe update on vectored read path
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@VladLazar
VladLazar committed Apr 23, 2024
1 parent cf9c6c3 commit 1826b13
Showing 1 changed file with 46 additions and 44 deletions.
90 changes: 46 additions & 44 deletions pageserver/src/tenant/timeline.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3250,55 +3250,57 @@ impl Timeline {
unmapped_keyspace.remove_overlapping_with(&keys_done_last_step);
completed_keyspace.merge(&keys_done_last_step);

let guard = timeline.layers.read().await;
let layers = guard.layer_map();
if unmapped_keyspace.start().is_some() {
let guard = timeline.layers.read().await;
let layers = guard.layer_map();

let in_memory_layer = layers.find_in_memory_layer(|l| {
let start_lsn = l.get_lsn_range().start;
cont_lsn > start_lsn
});
let in_memory_layer = layers.find_in_memory_layer(|l| {
let start_lsn = l.get_lsn_range().start;
cont_lsn > start_lsn
});

match in_memory_layer {
Some(l) => {
let lsn_range = l.get_lsn_range().start..cont_lsn;
fringe.update(
ReadableLayer::InMemoryLayer(l),
unmapped_keyspace.clone(),
lsn_range,
);
}
None => {
for range in unmapped_keyspace.ranges.iter() {
let results = layers.range_search(range.clone(), cont_lsn);

results
.found
.into_iter()
.map(|(SearchResult { layer, lsn_floor }, keyspace_accum)| {
(
ReadableLayer::PersistentLayer(guard.get_from_desc(&layer)),
keyspace_accum.to_keyspace(),
lsn_floor..cont_lsn,
)
})
.for_each(|(layer, keyspace, lsn_range)| {
fringe.update(layer, keyspace, lsn_range)
});
match in_memory_layer {
Some(l) => {
let lsn_range = l.get_lsn_range().start..cont_lsn;
fringe.update(
ReadableLayer::InMemoryLayer(l),
unmapped_keyspace.clone(),
lsn_range,
);
}
None => {
for range in unmapped_keyspace.ranges.iter() {
let results = layers.range_search(range.clone(), cont_lsn);

results
.found
.into_iter()
.map(|(SearchResult { layer, lsn_floor }, keyspace_accum)| {
(
ReadableLayer::PersistentLayer(guard.get_from_desc(&layer)),
keyspace_accum.to_keyspace(),
lsn_floor..cont_lsn,
)
})
.for_each(|(layer, keyspace, lsn_range)| {
fringe.update(layer, keyspace, lsn_range)
});
}
}
}
}

// It's safe to drop the layer map lock after planning the next round of reads.
// The fringe keeps readable handles for the layers which are safe to read even
// if layers were compacted or flushed.
//
// The more interesting consideration is: "Why is the read algorithm still correct
// if the layer map changes while it is operating?". Doing a vectored read on a
// timeline boils down to pushing an imaginary lsn boundary downwards for each range
// covered by the read. The layer map tells us how to move the lsn downwards for a
// range at *a particular point in time*. It is fine for the answer to be different
// at two different time points.
drop(guard);
// It's safe to drop the layer map lock after planning the next round of reads.
// The fringe keeps readable handles for the layers which are safe to read even
// if layers were compacted or flushed.
//
// The more interesting consideration is: "Why is the read algorithm still correct
// if the layer map changes while it is operating?". Doing a vectored read on a
// timeline boils down to pushing an imaginary lsn boundary downwards for each range
// covered by the read. The layer map tells us how to move the lsn downwards for a
// range at *a particular point in time*. It is fine for the answer to be different
// at two different time points.
drop(guard);
}

if let Some((layer_to_read, keyspace_to_read, lsn_range)) = fringe.next_layer() {
let next_cont_lsn = lsn_range.start;
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