From 7a386ee4430cbab929f3abc294669a5f197e07ce Mon Sep 17 00:00:00 2001
From: Marcus Holland-Moritz <github@mhxnet.de>
Date: Mon, 18 Aug 2025 18:59:29 +0200
Subject: [PATCH] chore: add ability to override hardware concurrency from
 environment

---
 .docker/build-linux.sh | 2 ++
 src/util.cpp           | 9 ++++++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/.docker/build-linux.sh b/.docker/build-linux.sh
index 1f896f81..bdc3910c 100755
--- a/.docker/build-linux.sh
+++ b/.docker/build-linux.sh
@@ -361,6 +361,8 @@ if [[ "-$BUILD_TYPE-" == *-static-* ]]; then
 
   if [[ -n "$CROSS_ARCH" ]]; then
     CMAKE_ARGS="${CMAKE_ARGS} -DCMAKE_SYSTEM_NAME=Linux -DCMAKE_SYSTEM_PROCESSOR=$_MARCH -DCMAKE_CROSSCOMPILING_EMULATOR=/usr/bin/qemu-$_MARCH -DFOLLY_HAVE_UNALIGNED_ACCESS=OFF -DFOLLY_HAVE_WEAK_SYMBOLS=ON -DFOLLY_HAVE_LINUX_VDSO=OFF -DFOLLY_HAVE_WCHAR_SUPPORT=OFF -DHAVE_VSNPRINTF_ERRORS=OFF"
+    # Limit emulated parallelism to 4 threads, otherwise the slowdown is substantial
+    export DWARFS_OVERRIDE_HARDWARE_CONCURRENCY=4
   fi
 fi
 
diff --git a/src/util.cpp b/src/util.cpp
index d9b8ca71..4b8b0ef0 100644
--- a/src/util.cpp
+++ b/src/util.cpp
@@ -424,7 +424,14 @@ std::string exception_str(std::exception_ptr const& e) {
 }
 
 unsigned int hardware_concurrency() noexcept {
-  return folly::hardware_concurrency();
+  static auto const env = [] {
+    std::optional<int> concurrency;
+    if (auto env = std::getenv("DWARFS_OVERRIDE_HARDWARE_CONCURRENCY")) {
+      concurrency = try_to<int>(env);
+    }
+    return concurrency;
+  }();
+  return env.value_or(folly::hardware_concurrency());
 }
 
 int get_current_umask() {