add ability to limit concurrency (#565)

effective concurrency in zenserver can be limited via the `--corelimit=<N>` option on the command line. Any value passed in here will be used instead of the return value from `std::thread::hardware_concurrency()` if it is lower.

* added --corelimit option to zenserver
* made sure thread pools are configured lazily and not during global init
* added log output indicating effective and HW concurrency
* added change log entry
* removed debug logging from ZenEntryPoint::Run()

also removed main thread naming on Linux since it makes the output from `top` and similar tools confusing (it shows `main` instead of `zenserver`)

5 files changed, 13 insertions, 2 deletions

diff --git a/src/zenserver/cache/httpstructuredcache.cpp b/src/zenserver/cache/httpstructuredcache.cpp
index dd5bf05cb..9f87c208c 100644
--- a/src/zenserver/cache/httpstructuredcache.cpp
+++ b/src/zenserver/cache/httpstructuredcache.cpp
@@ -423,7 +423,7 @@ HttpStructuredCacheService::HandleRequest(HttpServerRequest& Request)
 
 		std::string RecordPath = UrlDecode(Params.GetValue("path"));
 
-		uint32_t ThreadCount = std::thread::hardware_concurrency();
+		uint32_t ThreadCount = GetHardwareConcurrency();
 		if (auto Param = Params.GetValue("thread_count"); Param.empty() == false)
 		{
 			if (auto Value = ParseInt<uint64_t>(Param))
diff --git a/src/zenserver/config.cpp b/src/zenserver/config.cpp
index 0cf5a9ca3..697d44214 100644
--- a/src/zenserver/config.cpp
+++ b/src/zenserver/config.cpp
@@ -817,6 +817,7 @@ ParseCliOptions(int argc, char* argv[], ZenServerOptions& ServerOptions)
 	options.add_options()("malloc",
 						  "Configure memory allocator subsystem",
 						  cxxopts::value(ServerOptions.MemoryOptions)->default_value("mimalloc"));
+	options.add_options()("corelimit", "Limit concurrency", cxxopts::value(ServerOptions.CoreLimit));
 
 	// clang-format off
 	options.add_options("logging")
diff --git a/src/zenserver/config.h b/src/zenserver/config.h
index 8380e72e7..3f7cb149a 100644
--- a/src/zenserver/config.h
+++ b/src/zenserver/config.h
@@ -207,6 +207,7 @@ struct ZenServerOptions
 	bool					 ObjectStoreEnabled = false;
 	bool					 NoConsoleOutput	= false;  // Control default use of stdout for diagnostics
 	bool					 QuietConsole		= false;  // Configure console logger output to level WARN
+	int						 CoreLimit			= 0;	  // If set, hardware concurrency queries are capped at this number
 	std::string				 Loggers[zen::logging::level::LogLevelCount];
 	std::string				 ScrubOptions;
 #if ZEN_WITH_TRACE
diff --git a/src/zenserver/main.cpp b/src/zenserver/main.cpp
index a91c95ffb..b4d53ec5f 100644
--- a/src/zenserver/main.cpp
+++ b/src/zenserver/main.cpp
@@ -93,8 +93,13 @@ ZenEntryPoint::ZenEntryPoint(ZenServerOptions& ServerOptions) : m_ServerOptions(
 int
 ZenEntryPoint::Run()
 {
-	ZEN_INFO("ZenEntryPoint::Run()");
+	// On Linux this has the unfortunate side effect of making `top` and other tools display
+	// `main` as the program name since threads and processes have a closer relationship
+	// there. So we don't name the main thread explicitly there.
+
+#ifndef ZEN_PLATFORM_LINUX
 	zen::SetCurrentThreadName("main");
+#endif
 
 #if ZEN_USE_SENTRY
 	SentryIntegration Sentry;
@@ -444,6 +449,8 @@ main(int argc, char* argv[])
 #endif
 		}
 
+		LimitHardwareConcurrency(ServerOptions.CoreLimit);
+
 		std::string_view DeleteReason;
 
 		if (ServerOptions.IsCleanStart)
diff --git a/src/zenserver/zenserver.cpp b/src/zenserver/zenserver.cpp
index cab234165..975459e68 100644
--- a/src/zenserver/zenserver.cpp
+++ b/src/zenserver/zenserver.cpp
@@ -244,6 +244,8 @@ ZenServer::Initialize(const ZenServerOptions& ServerOptions, ZenServerState::Zen
 
 	m_StatusService.RegisterHandler("status", *this);
 
+	ZEN_INFO("Effective concurrency: {} (hw: {})", GetHardwareConcurrency(), std::thread::hardware_concurrency());
+
 	// Initialize storage and services
 
 	ZEN_INFO("initializing storage");