From 115549261c0506f79987fc99366aaa53548a7961 Mon Sep 17 00:00:00 2001
From: Bojan Serafimov <bojan.serafimov7@gmail.com>
Date: Thu, 5 Jan 2023 01:58:18 -0500
Subject: [PATCH] Add get_difficulty_map method

---
 pageserver/benches/bench_layer_map.rs | 18 ++++++++++++++----
 pageserver/src/tenant/layer_map.rs    | 12 ++++++++++++
 2 files changed, 26 insertions(+), 4 deletions(-)

diff --git a/pageserver/benches/bench_layer_map.rs b/pageserver/benches/bench_layer_map.rs
index 0e273e0cdc..81f37d12ac 100644
--- a/pageserver/benches/bench_layer_map.rs
+++ b/pageserver/benches/bench_layer_map.rs
@@ -125,14 +125,19 @@ fn bench_from_real_project(c: &mut Criterion) {
     let queries: Vec<(Key, Lsn)> = uniform_query_pattern(&layer_map);
 
     // Define and name the benchmark function
-    let mut group = c.benchmark_group("real_map_uniform_queries");
-    group.bench_function("current_code", |b| {
+    let mut group = c.benchmark_group("real_map");
+    group.bench_function("uniform_queries", |b| {
         b.iter(|| {
             for q in queries.clone().into_iter() {
                 layer_map.search(q.0, q.1);
             }
         });
     });
+    group.bench_function("get_difficulty_map", |b| {
+        b.iter(|| {
+            // TODO call get_difficulty_map
+        });
+    });
     group.finish();
 }
 
@@ -168,14 +173,19 @@ fn bench_sequential(c: &mut Criterion) {
         .collect();
 
     // Define and name the benchmark function
-    let mut group = c.benchmark_group("sequential_uniform_queries");
-    group.bench_function("current_code", |b| {
+    let mut group = c.benchmark_group("sequential");
+    group.bench_function("uniform_queries", |b| {
         b.iter(|| {
             for q in queries.clone().into_iter() {
                 layer_map.search(q.0, q.1);
             }
         });
     });
+    group.bench_function("get_difficulty_map", |b| {
+        b.iter(|| {
+            // TODO call get_difficulty_map
+        });
+    });
     group.finish();
 }
 
diff --git a/pageserver/src/tenant/layer_map.rs b/pageserver/src/tenant/layer_map.rs
index 361b0808a0..63a6019c60 100644
--- a/pageserver/src/tenant/layer_map.rs
+++ b/pageserver/src/tenant/layer_map.rs
@@ -10,6 +10,7 @@
 //! corresponding files are written to disk.
 //!
 
+use crate::keyspace::KeyPartitioning;
 use crate::metrics::NUM_ONDISK_LAYERS;
 use crate::repository::Key;
 use crate::tenant::storage_layer::InMemoryLayer;
@@ -321,6 +322,17 @@ where
         Ok(max_stacked_deltas)
     }
 
+    /// For each part of a keyspace partitioning, return the maximum number of layers
+    /// that would be needed for page reconstruction in that part at the given LSN.
+    ///
+    /// This method is used to decide where to create new image layers. Computing the
+    /// result for the entire partitioning at once allows this function to be more
+    /// efficient, and further optimization is possible by using iterators instead,
+    /// to allow early return.
+    pub fn get_difficulty_map(&self, _lsn: Lsn, _partitioning: &KeyPartitioning) -> Vec<usize> {
+        todo!()
+    }
+
     /// Return all L0 delta layers
     pub fn get_level0_deltas(&self) -> Result<Vec<Arc<L>>> {
         Ok(self.l0_delta_layers.clone())