pageserver: set a concurrency limit for LocalFS (#10676)

## Problem

The local filesystem backend for remote storage doesn't set a
concurrency limit. While it can't/won't enforce a concurrency limit
itself, this also bounds the upload queue concurrency. Some tests create
thousands of uploads, which slows down the quadratic scheduling of the
upload queue, and there is no point spawning that many Tokio tasks.

Resolves #10409.

## Summary of changes

Set a concurrency limit of 100 for the LocalFS backend.

Before: `test_layer_map[release-pg17].test_query: 68.338 s`
After: `test_layer_map[release-pg17].test_query: 5.209 s`

libs/remote_storage/src/config.rs

@@ -7,7 +7,7 @@ use serde::{Deserialize, Serialize};
 
 use crate::{
     DEFAULT_MAX_KEYS_PER_LIST_RESPONSE, DEFAULT_REMOTE_STORAGE_AZURE_CONCURRENCY_LIMIT,
-    DEFAULT_REMOTE_STORAGE_S3_CONCURRENCY_LIMIT,
+    DEFAULT_REMOTE_STORAGE_LOCALFS_CONCURRENCY_LIMIT, DEFAULT_REMOTE_STORAGE_S3_CONCURRENCY_LIMIT,
 };
 
 /// External backup storage configuration, enough for creating a client for that storage.
@@ -45,11 +45,11 @@ impl RemoteStorageKind {
 
 impl RemoteStorageConfig {
     /// Helper to fetch the configured concurrency limit.
-    pub fn concurrency_limit(&self) -> Option<usize> {
+    pub fn concurrency_limit(&self) -> usize {
         match &self.storage {
-            RemoteStorageKind::LocalFs { .. } => None,
-            RemoteStorageKind::AwsS3(c) => Some(c.concurrency_limit.into()),
-            RemoteStorageKind::AzureContainer(c) => Some(c.concurrency_limit.into()),
+            RemoteStorageKind::LocalFs { .. } => DEFAULT_REMOTE_STORAGE_LOCALFS_CONCURRENCY_LIMIT,
+            RemoteStorageKind::AwsS3(c) => c.concurrency_limit.into(),
+            RemoteStorageKind::AzureContainer(c) => c.concurrency_limit.into(),
         }
     }
 }

libs/remote_storage/src/lib.rs

@@ -65,6 +65,12 @@ pub const DEFAULT_REMOTE_STORAGE_S3_CONCURRENCY_LIMIT: usize = 100;
 /// Here, a limit of max 20k concurrent connections was noted.
 /// <https://learn.microsoft.com/en-us/answers/questions/1301863/is-there-any-limitation-to-concurrent-connections>
 pub const DEFAULT_REMOTE_STORAGE_AZURE_CONCURRENCY_LIMIT: usize = 100;
+/// Set this limit analogously to the S3 limit.
+///
+/// The local filesystem backend doesn't enforce a concurrency limit itself, but this also bounds
+/// the upload queue concurrency. Some tests create thousands of uploads, which slows down the
+/// quadratic scheduling of the upload queue, and there is no point spawning so many Tokio tasks.
+pub const DEFAULT_REMOTE_STORAGE_LOCALFS_CONCURRENCY_LIMIT: usize = 100;
 /// No limits on the client side, which currenltly means 1000 for AWS S3.
 /// <https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListObjectsV2.html#API_ListObjectsV2_RequestSyntax>
 pub const DEFAULT_MAX_KEYS_PER_LIST_RESPONSE: Option<i32> = None;

pageserver/src/tenant/remote_timeline_client.rs

@@ -437,8 +437,7 @@ impl RemoteTimelineClient {
             .conf
             .remote_storage_config
             .as_ref()
-            .and_then(|r| r.concurrency_limit())
-            .unwrap_or(0);
+            .map_or(0, |r| r.concurrency_limit());
         let mut upload_queue = self.upload_queue.lock().unwrap();
         upload_queue.initialize_with_current_remote_index_part(index_part, inprogress_limit)?;
         self.update_remote_physical_size_gauge(Some(index_part));
@@ -461,8 +460,7 @@ impl RemoteTimelineClient {
             .conf
             .remote_storage_config
             .as_ref()
-            .and_then(|r| r.concurrency_limit())
-            .unwrap_or(0);
+            .map_or(0, |r| r.concurrency_limit());
         let mut upload_queue = self.upload_queue.lock().unwrap();
         upload_queue.initialize_empty_remote(local_metadata, inprogress_limit)?;
         self.update_remote_physical_size_gauge(None);
@@ -484,8 +482,7 @@ impl RemoteTimelineClient {
             .conf
             .remote_storage_config
             .as_ref()
-            .and_then(|r| r.concurrency_limit())
-            .unwrap_or(0);
+            .map_or(0, |r| r.concurrency_limit());
 
         let mut upload_queue = self.upload_queue.lock().unwrap();
         upload_queue.initialize_with_current_remote_index_part(index_part, inprogress_limit)?;