pageserver: reduce default compaction_upper_limit to 20 (#10889)

## Problem

We've seen the previous default of 50 cause OOMs. Compacting many L0
layers at once now has limited benefit, since the cost is mostly linear
anyway. This is already being reduced to 20 in production settings.

## Summary of changes

Reduce `DEFAULT_COMPACTION_UPPER_LIMIT` to 20.

Once released, let's remove the config overrides.

libs/pageserver_api/src/config.rs

@@ -544,10 +544,11 @@ pub mod tenant_conf_defaults {
     pub const DEFAULT_COMPACTION_PERIOD: &str = "20 s";
     pub const DEFAULT_COMPACTION_THRESHOLD: usize = 10;
 
-    // This value needs to be tuned to avoid OOM. We have 3/4 of the total CPU threads to do background works, that's 16*3/4=9 on
-    // most of our pageservers. Compaction ~50 layers requires about 2GB memory (could be reduced later by optimizing L0 hole
-    // calculation to avoid loading all keys into the memory). So with this config, we can get a maximum peak compaction usage of 18GB.
-    pub const DEFAULT_COMPACTION_UPPER_LIMIT: usize = 50;
+    // This value needs to be tuned to avoid OOM. We have 3/4*CPUs threads for L0 compaction, that's
+    // 3/4*16=9 on most of our pageservers. Compacting 20 layers requires about 1 GB memory (could
+    // be reduced later by optimizing L0 hole calculation to avoid loading all keys into memory). So
+    // with this config, we can get a maximum peak compaction usage of 9 GB.
+    pub const DEFAULT_COMPACTION_UPPER_LIMIT: usize = 20;
     pub const DEFAULT_COMPACTION_L0_FIRST: bool = false;
     pub const DEFAULT_COMPACTION_L0_SEMAPHORE: bool = true;