[clang-tools-extra] [clangd][unittests] Limit paralelism for clangd unittests (PR #65444)

[kadir çetinkaya via cfe-commits]

https://github.com/kadircet created https://github.com/llvm/llvm-project/pull/65444:

None

>From 0b33a1277beb1ec518cb000923d02d78365bfd82 Mon Sep 17 00:00:00 2001
From: Kadir Cetinkaya <kadircet at google.com>
Date: Tue, 5 Sep 2023 19:19:47 +0200
Subject: [PATCH] [clangd][unittests] Limit paralelism for clangd unittests

We started seeing a lot of timeouts that align with the change in lit to
execute gtests in shards. The logic there assumes tests are
single-threaded, which is the case for most of the LLVM, hence they
pick #shards ~ #cores (by slightly overshooting).

There are enough unittests in clangd that rely on multi-threading, they
can create arbitrarily many threads but we limit amount of meaningful
work to ~4 thread per process.

This change ensures that we're accounting for that paralelism when
executing clangd tests and not overloading test executors.

In theory the change overestimates the requirements, not all tests are
multi-threaded, but it doesn't seem to be resulting in any regressions
on my local runs.

Fixes https://github.com/llvm/llvm-project/issues/64964.
Fixes https://github.com/clangd/clangd/issues/1712.
---
 clang-tools-extra/clangd/unittests/lit.cfg.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/clang-tools-extra/clangd/unittests/lit.cfg.py b/clang-tools-extra/clangd/unittests/lit.cfg.py
index 48ee5f5d5ab9203..33aa9e61f4ce9dd 100644
--- a/clang-tools-extra/clangd/unittests/lit.cfg.py
+++ b/clang-tools-extra/clangd/unittests/lit.cfg.py
@@ -1,4 +1,5 @@
 import lit.formats
+import lit.util
 
 config.name = "Clangd Unit Tests"
 config.test_format = lit.formats.GoogleTest(".", "Tests")
@@ -9,6 +10,13 @@
 # FIXME: it seems every project has a copy of this logic. Move it somewhere.
 import platform
 
+# Clangd unittests uses ~4 threads per test. So make sure we don't over commit.
+core_count = lit.util.usable_core_count()
+# FIXME: Split unittests into groups that use threads, and groups that do not,
+# and only limit multi-threaded tests.
+lit_config.parallelism_groups["clangd"] = max(1, core_count // 4)
+config.parallelism_group = "clangd"
+
 if platform.system() == "Darwin":
     shlibpath_var = "DYLD_LIBRARY_PATH"
 elif platform.system() == "Windows":