tdf#98955 hardware_concurrency not ideal for thread pools

A new static member getPreferredConcurrency added to
comphelper::ThreadPool to return a configurable max
number of threads.

By default the new function returns the hardware_concurrency
value provided by std::thread. When MAX_CONCURRENCY envar is
defined, the return value is limited to whatever is set there.

Three call-sites that used std::thread::hardware_concurrency
have been replaced with getPreferredConcurrency.

Unittests added to cover the functionality of the new member.

Unittests are capped to 4 threads.

Change-Id: I3332e393a88a5ed436316fa712ed920a4b37f4af
Reviewed-on: https://gerrit.libreoffice.org/26254
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Ashod Nakashian <ashnakash@gmail.com>

1 files changed, 23 insertions, 1 deletions

diff --git a/comphelper/source/misc/threadpool.cxx b/comphelper/source/misc/threadpool.cxx
index 58613649e8fa..8680e00e6e34 100644
--- a/comphelper/source/misc/threadpool.cxx
+++ b/comphelper/source/misc/threadpool.cxx
@@ -10,6 +10,7 @@
 #include <comphelper/threadpool.hxx>
 
 #include <rtl/instance.hxx>
+#include <rtl/string.hxx>
 #include <algorithm>
 #include <memory>
 #include <thread>
@@ -115,7 +116,7 @@ struct ThreadPoolStatic : public rtl::StaticWithInit< std::shared_ptr< ThreadPoo
                                                       ThreadPoolStatic >
 {
     std::shared_ptr< ThreadPool > operator () () {
-        sal_Int32 nThreads = std::max( std::thread::hardware_concurrency(), 1U );
+        const sal_Int32 nThreads = ThreadPool::getPreferredConcurrency();
         return std::make_shared< ThreadPool >( nThreads );
     };
 };
@@ -125,6 +126,27 @@ ThreadPool& ThreadPool::getSharedOptimalPool()
     return *ThreadPoolStatic::get().get();
 }
 
+sal_Int32 ThreadPool::getPreferredConcurrency()
+{
+    static sal_Int32 ThreadCount = 0;
+    if (ThreadCount == 0)
+    {
+        const sal_Int32 nHardThreads = std::max(std::thread::hardware_concurrency(), 1U);
+        sal_Int32 nThreads = nHardThreads;
+        const char *pEnv = getenv("MAX_CONCURRENCY");
+        if (pEnv != nullptr)
+        {
+            // Override with user/admin preferrence.
+            nThreads = rtl_str_toInt32(pEnv, 10);
+        }
+
+        nThreads = std::min(nHardThreads, nThreads);
+        ThreadCount = std::max<sal_Int32>(nThreads, 1);
+    }
+
+    return ThreadCount;
+}
+
 void ThreadPool::waitAndCleanupWorkers()
 {
     waitUntilEmpty();