pageserver: rate limit log for loads of layers visited (#8228)

## Problem
At high percentiles we see more than 800 layers being visited by the
read path. We need the tenant/timeline to investigate.

## Summary of changes
Add a rate limited log line when the average number of layers visited
per key is in the last specified histogram bucket.
I plan to use this to identify tenants in us-east-2 staging that exhibit
this behaviour. Will revert before next week's release.

libs/pageserver_api/src/keyspace.rs

@@ -17,6 +17,16 @@ pub struct KeySpace {
     pub ranges: Vec<Range<Key>>,
 }
 
+impl std::fmt::Display for KeySpace {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "[")?;
+        for range in &self.ranges {
+            write!(f, "{}..{},", range.start, range.end)?;
+        }
+        write!(f, "]")
+    }
+}
+
 /// A wrapper type for sparse keyspaces.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
 pub struct SparseKeySpace(pub KeySpace);

pageserver/src/tenant/timeline.rs

@@ -996,6 +996,7 @@ impl Timeline {
     }
 
     pub(crate) const MAX_GET_VECTORED_KEYS: u64 = 32;
+    pub(crate) const VEC_GET_LAYERS_VISITED_WARN_THRESH: f64 = 512.0;
 
     /// Look up multiple page versions at a given LSN
     ///
@@ -1228,7 +1229,7 @@ impl Timeline {
         let get_data_timer = crate::metrics::GET_RECONSTRUCT_DATA_TIME
             .for_get_kind(get_kind)
             .start_timer();
-        self.get_vectored_reconstruct_data(keyspace, lsn, reconstruct_state, ctx)
+        self.get_vectored_reconstruct_data(keyspace.clone(), lsn, reconstruct_state, ctx)
             .await?;
         get_data_timer.stop_and_record();
 
@@ -1258,11 +1259,26 @@ impl Timeline {
         // (this is a requirement, not a bug). Skip updating the metric in these cases
         // to avoid infinite results.
         if !results.is_empty() {
+            let avg = layers_visited as f64 / results.len() as f64;
+            if avg >= Self::VEC_GET_LAYERS_VISITED_WARN_THRESH {
+                use utils::rate_limit::RateLimit;
+                static LOGGED: Lazy<Mutex<RateLimit>> =
+                    Lazy::new(|| Mutex::new(RateLimit::new(Duration::from_secs(10))));
+                let mut rate_limit = LOGGED.lock().unwrap();
+                rate_limit.call(|| {
+                    tracing::info!(
+                    tenant_id = %self.tenant_shard_id.tenant_id,
+                    shard_id = %self.tenant_shard_id.shard_slug(),
+                    timeline_id = %self.timeline_id,
+                    "Vectored read for {} visited {} layers on average per key and {} in total. {}/{} pages were returned",
+                    keyspace, avg, layers_visited, results.len(), keyspace.total_raw_size());
+                });
+            }
+
             // Note that this is an approximation. Tracking the exact number of layers visited
             // per key requires virtually unbounded memory usage and is inefficient
             // (i.e. segment tree tracking each range queried from a layer)
-            crate::metrics::VEC_READ_NUM_LAYERS_VISITED
-                .observe(layers_visited as f64 / results.len() as f64);
+            crate::metrics::VEC_READ_NUM_LAYERS_VISITED.observe(avg);
         }
 
         Ok(results)