From 4be58522fbd61e74e7fead19d106c8182f1cefba Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Mon, 29 Jul 2024 16:49:22 +0200
Subject: [PATCH] l0_flush: use mode=direct by default => coverage in automated
 tests (#8534)

Testing in staging and pre-prod has been [going

well](https://github.com/neondatabase/neon/issues/7418#issuecomment-2255474917).

This PR enables mode=direct by default, thereby providing additional
coverage in the automated tests:
- Rust tests
- Integration tests
- Nightly pagebench (likely irrelevant because it's read-only)

Production deployments continue to use `mode=page-cache` for the time
being: https://github.com/neondatabase/aws/pull/1655

refs https://github.com/neondatabase/neon/issues/7418
---
 pageserver/src/l0_flush.rs | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/pageserver/src/l0_flush.rs b/pageserver/src/l0_flush.rs
index 7fe8fedc63..8945e5accd 100644
--- a/pageserver/src/l0_flush.rs
+++ b/pageserver/src/l0_flush.rs
@@ -2,13 +2,23 @@ use std::{num::NonZeroUsize, sync::Arc};
 
 use crate::tenant::ephemeral_file;
 
-#[derive(Default, Debug, PartialEq, Eq, Clone, serde::Deserialize)]
+#[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize)]
 #[serde(tag = "mode", rename_all = "kebab-case", deny_unknown_fields)]
 pub enum L0FlushConfig {
-    #[default]
     PageCached,
     #[serde(rename_all = "snake_case")]
-    Direct { max_concurrency: NonZeroUsize },
+    Direct {
+        max_concurrency: NonZeroUsize,
+    },
+}
+
+impl Default for L0FlushConfig {
+    fn default() -> Self {
+        Self::Direct {
+            // TODO: using num_cpus results in different peak memory usage on different instance types.
+            max_concurrency: NonZeroUsize::new(usize::max(1, num_cpus::get())).unwrap(),
+        }
+    }
 }
 
 #[derive(Clone)]