fix(testing): Use 1 MB shared_buffers even with LFC

Everything smaller might be not enough for Postgres under load,
and can cause errors like 'no unpinned buffers available'.

See Konstantin's comment [1] as well.

Fixes #9956

[1]: #9956 (comment)

control_plane/src/endpoint.rs

@@ -310,6 +310,10 @@ impl Endpoint {
         conf.append("wal_log_hints", "off");
         conf.append("max_replication_slots", "10");
         conf.append("hot_standby", "on");
+        // Set to 1MB to both exercise getPage requests/LFC, and still have enough room for
+        // Postgres to operate. Everything smaller might be not enough for Postgres under load,
+        // and can cause errors like 'no unpinned buffers available', see
+        // <https://github.com/neondatabase/neon/issues/9956>
         conf.append("shared_buffers", "1MB");
         conf.append("fsync", "off");
         conf.append("max_connections", "100");

test_runner/fixtures/neon_fixtures.py

@@ -3801,13 +3801,11 @@ def create(
                     assert size_to_bytes(size) >= size_to_bytes(
                         "1MB"
                     ), "LFC size cannot be set less than 1MB"
-            # shared_buffers = 512kB to make postgres use LFC intensively
-            # neon.max_file_cache_size and neon.file_cache size limit are
-            # set to 1MB because small LFC is better for testing (helps to find more problems)
             lfc_path_escaped = str(lfc_path).replace("'", "''")
             config_lines = [
-                "shared_buffers = 512kB",
                 f"neon.file_cache_path = '{lfc_path_escaped}'",
+                # neon.max_file_cache_size and neon.file_cache size limits are
+                # set to 1MB because small LFC is better for testing (helps to find more problems)
                 "neon.max_file_cache_size = 1MB",
                 "neon.file_cache_size_limit = 1MB",
             ] + config_lines