Tiered compaction: improvements to the windows (#7787)

Tiered compaction employs two sliding windows over the keyspace:
`KeyspaceWindow` for the image layer generation and `Window` for the
delta layer generation. Do some fixes to both windows:

* The distinction between the two windows is not very clear. Do the
absolute minimum to mention where they are used in the rustdoc
description of the struct. Maybe we should rename them (say
`WindowForImage` and `WindowForDelta`) or merge them into one window
implementation.
* Require the keys to strictly increase. The `accum_key_values` already
combines the key, so there is no logic needed in `Window::feed` for the
same key repeating. This is a follow-up to address the request in
#7671 (review)
* In `choose_next_delta`, we claimed in the comment to use 1.25 as the
factor but it was 1.66 instead. Fix this discrepancy by using `*5/4` as
the two operations.

pageserver/compaction/src/compact_tiered.rs

@@ -663,8 +663,8 @@ where
     }
 }
 
-// Sliding window through keyspace and values
-// This is used by over_with_images to decide on good split points
+/// Sliding window through keyspace and values for image layer
+/// This is used by [`LevelCompactionState::cover_with_images`] to decide on good split points
 struct KeyspaceWindow<K> {
     head: KeyspaceWindowHead<K>,
 
@@ -804,9 +804,9 @@ struct WindowElement<K> {
     accum_size: u64,
 }
 
-// Sliding window through keyspace and values
-//
-// This is used to decide what layer to write next, from the beginning of the window.
+/// Sliding window through keyspace and values for delta layer tiling
+///
+/// This is used to decide which delta layer to write next.
 struct Window<K> {
     elems: VecDeque<WindowElement<K>>,
 
@@ -830,11 +830,13 @@ where
     fn feed(&mut self, key: K, size: u64) {
         let last_size;
         if let Some(last) = self.elems.back_mut() {
-            assert!(last.last_key <= key);
-            if key == last.last_key {
-                last.accum_size += size;
-                return;
-            }
+            // We require the keys to be strictly increasing for the window.
+            // Keys should already have been deduplicated by `accum_key_values`
+            assert!(
+                last.last_key < key,
+                "last_key(={}) >= key(={key})",
+                last.last_key
+            );
             last_size = last.accum_size;
         } else {
             last_size = 0;
@@ -922,7 +924,7 @@ where
         // If we're willing to stretch it up to 1.25 target size, could we
         // gobble up the rest of the work? This avoids creating very small
         // "tail" layers at the end of the keyspace
-        if !has_more && self.remain_size() < target_size * 5 / 3 {
+        if !has_more && self.remain_size() < target_size * 5 / 4 {
             self.commit_upto(self.elems.len());
         } else {
             let delta_split_at = self.find_size_split(target_size);