3 files changed, 77 insertions, 45 deletions

diff --git a/src/cpp/server/server_cc.cc b/src/cpp/server/server_cc.cc
index 6e6e0bfffe..786ef44e3e 100644
--- a/src/cpp/server/server_cc.cc
+++ b/src/cpp/server/server_cc.cc
@@ -47,6 +47,12 @@
 namespace grpc {
 namespace {
 
+// The default value for maximum number of threads that can be created in the
+// sync server. This value of 1500 is empirically chosen. To increase the max
+// number of threads in a sync server, pass a custom ResourceQuota object (with
+// the desired number of max-threads set) to the server builder
+#define DEFAULT_MAX_SYNC_SERVER_THREADS 1500
+
 class DefaultGlobalCallbacks final : public Server::GlobalCallbacks {
  public:
   ~DefaultGlobalCallbacks() override {}
@@ -395,7 +401,9 @@ Server::Server(
   if (sync_server_cqs_ != nullptr) {
     bool default_rq_created = false;
     if (server_rq == nullptr) {
-      server_rq = grpc_resource_quota_create("SyncServer-Default");
+      server_rq = grpc_resource_quota_create("SyncServer-default-rq");
+      grpc_resource_quota_set_max_threads(server_rq,
+                                          DEFAULT_MAX_SYNC_SERVER_THREADS);
       default_rq_created = true;
     }
 
diff --git a/src/cpp/thread_manager/thread_manager.cc b/src/cpp/thread_manager/thread_manager.cc
index c0fa98798a..5d367511e2 100644
--- a/src/cpp/thread_manager/thread_manager.cc
+++ b/src/cpp/thread_manager/thread_manager.cc
@@ -22,8 +22,8 @@
 #include <mutex>
 
 #include <grpc/support/log.h>
-
 #include "src/core/lib/gprpp/thd.h"
+#include "src/core/lib/iomgr/exec_ctx.h"
 
 namespace grpc {
 
@@ -55,7 +55,8 @@ ThreadManager::ThreadManager(const char* name,
       num_pollers_(0),
       min_pollers_(min_pollers),
       max_pollers_(max_pollers == -1 ? INT_MAX : max_pollers),
-      num_threads_(0) {
+      num_threads_(0),
+      max_active_threads_sofar_(0) {
   resource_user_ = grpc_resource_user_create(resource_quota, name);
 }
 
@@ -65,6 +66,7 @@ ThreadManager::~ThreadManager() {
     GPR_ASSERT(num_threads_ == 0);
   }
 
+  grpc_core::ExecCtx exec_ctx;  // grpc_resource_user_unref needs an exec_ctx
   grpc_resource_user_unref(resource_user_);
   CleanupCompletedThreads();
 }
@@ -86,17 +88,27 @@ bool ThreadManager::IsShutdown() {
   return shutdown_;
 }
 
+int ThreadManager::GetMaxActiveThreadsSoFar() {
+  std::lock_guard<std::mutex> list_lock(list_mu_);
+  return max_active_threads_sofar_;
+}
+
 void ThreadManager::MarkAsCompleted(WorkerThread* thd) {
   {
     std::lock_guard<std::mutex> list_lock(list_mu_);
     completed_threads_.push_back(thd);
   }
 
-  std::lock_guard<std::mutex> lock(mu_);
-  num_threads_--;
-  if (num_threads_ == 0) {
-    shutdown_cv_.notify_one();
+  {
+    std::lock_guard<std::mutex> lock(mu_);
+    num_threads_--;
+    if (num_threads_ == 0) {
+      shutdown_cv_.notify_one();
+    }
   }
+
+  // Give a thread back to the resource quota
+  grpc_resource_user_free_threads(resource_user_, 1);
 }
 
 void ThreadManager::CleanupCompletedThreads() {
@@ -111,34 +123,24 @@ void ThreadManager::CleanupCompletedThreads() {
 }
 
 void ThreadManager::Initialize() {
+  if (!grpc_resource_user_alloc_threads(resource_user_, min_pollers_)) {
+    gpr_log(GPR_ERROR,
+            "No thread quota available to even create the minimum required "
+            "polling threads (i.e %d). Unable to start the thread manager",
+            min_pollers_);
+    abort();
+  }
+
   {
     std::unique_lock<std::mutex> lock(mu_);
     num_pollers_ = min_pollers_;
     num_threads_ = min_pollers_;
+    max_active_threads_sofar_ = min_pollers_;
   }
 
   for (int i = 0; i < min_pollers_; i++) {
-    if (!CreateNewThread(this)) {
-      gpr_log(GPR_ERROR,
-              "No quota available to create additional threads. Created %d (of "
-              "%d) threads",
-              i, min_pollers_);
-      break;
-    }
-  }
-}
-
-bool ThreadManager::CreateNewThread(ThreadManager* thd_mgr) {
-  if (!grpc_resource_user_alloc_threads(thd_mgr->resource_user_, 1)) {
-    return false;
+    new WorkerThread(this);
   }
-  // Create a new thread (which ends up calling the MainWorkLoop() function
-  new WorkerThread(thd_mgr);
-  return true;
-}
-
-void ThreadManager::ReleaseThread(ThreadManager* thd_mgr) {
-  grpc_resource_user_free_threads(thd_mgr->resource_user_, 1);
 }
 
 void ThreadManager::MainWorkLoop() {
@@ -162,14 +164,17 @@ void ThreadManager::MainWorkLoop() {
         done = true;
         break;
       case WORK_FOUND:
-        // If we got work and there are now insufficient pollers, start a new
-        // one
-        if (!shutdown_ && num_pollers_ < min_pollers_) {
+        // If we got work and there are now insufficient pollers and there is
+        // quota available to create a new thread,start a new poller thread
+        if (!shutdown_ && num_pollers_ < min_pollers_ &&
+            grpc_resource_user_alloc_threads(resource_user_, 1)) {
           num_pollers_++;
           num_threads_++;
+          max_active_threads_sofar_ =
+              std::max(max_active_threads_sofar_, num_threads_);
           // Drop lock before spawning thread to avoid contention
           lock.unlock();
-          CreateNewThread(this);
+          new WorkerThread(this);
         } else {
           // Drop lock for consistency with above branch
           lock.unlock();
@@ -219,10 +224,9 @@ void ThreadManager::MainWorkLoop() {
     }
   };
 
-  // This thread is exiting. Do some cleanup work (i.e delete already completed
-  // worker threads and also release 1 thread back to the resource quota)
+  // This thread is exiting. Do some cleanup work i.e delete already completed
+  // worker threads
   CleanupCompletedThreads();
-  ReleaseThread(this);
 
   // If we are here, either ThreadManager is shutting down or it already has
   // enough threads.
diff --git a/src/cpp/thread_manager/thread_manager.h b/src/cpp/thread_manager/thread_manager.h
index 23bd38ee4f..8332befed0 100644
--- a/src/cpp/thread_manager/thread_manager.h
+++ b/src/cpp/thread_manager/thread_manager.h
@@ -86,6 +86,11 @@ class ThreadManager {
   // all the threads have drained all the outstanding work
   virtual void Wait();
 
+  // Max number of concurrent threads that were ever active in this thread
+  // manager so far. This is useful for debugging purposes (and in unit tests)
+  // to check if resource_quota is properly being enforced.
+  int GetMaxActiveThreadsSoFar();
+
  private:
   // Helper wrapper class around grpc_core::Thread. Takes a ThreadManager object
   // and starts a new grpc_core::Thread to calls the Run() function.
@@ -93,6 +98,23 @@ class ThreadManager {
   // The Run() function calls ThreadManager::MainWorkLoop() function and once
   // that completes, it marks the WorkerThread completed by calling
   // ThreadManager::MarkAsCompleted()
+  //
+  // WHY IS THIS NEEDED?:
+  // When a thread terminates, some other tread *must* call Join() on that
+  // thread so that the resources are released. Having a WorkerThread wrapper
+  // will make this easier. Once Run() completes, each thread calls the
+  // following two functions:
+  //    ThreadManager::CleanupCompletedThreads()
+  //    ThreadManager::MarkAsCompleted()
+  //
+  //  - MarkAsCompleted() puts the WorkerThread object in the ThreadManger's
+  //    completed_threads_ list
+  //  - CleanupCompletedThreads() calls "Join()" on the threads that are already
+  //    in the completed_threads_ list  (since a thread cannot call Join() on
+  //    itself, it calls CleanupCompletedThreads() *before* calling
+  //    MarkAsCompleted())
+  // TODO: sreek - consider creating the threads 'detached' so that Join() need
+  // not be called
   class WorkerThread {
    public:
     WorkerThread(ThreadManager* thd_mgr);
@@ -113,15 +135,8 @@ class ThreadManager {
   void MarkAsCompleted(WorkerThread* thd);
   void CleanupCompletedThreads();
 
-  // Checks the resource quota and if available, creates a thread and returns
-  // true. If quota is not available, returns false (and thread is not created)
-  static bool CreateNewThread(ThreadManager* thd_mgr);
-
-  // Give back a thread to the resource quota
-  static void ReleaseThread(ThreadManager* thd_mgr);
-
-  // Protects shutdown_, num_pollers_ and num_threads_
-  // TODO: sreek - Change num_pollers and num_threads_ to atomics
+  // Protects shutdown_, num_pollers_, num_threads_ and
+  // max_active_threads_sofar_
   std::mutex mu_;
 
   bool shutdown_;
@@ -142,10 +157,15 @@ class ThreadManager {
   int min_pollers_;
   int max_pollers_;
 
-  // The total number of threads (includes threads includes the threads that are
-  // currently polling i.e num_pollers_)
+  // The total number of threads currently active (includes threads includes the
+  // threads that are currently polling i.e num_pollers_)
   int num_threads_;
 
+  // See GetMaxActiveThreadsSoFar()'s description.
+  // To be more specific, this variable tracks the max value num_threads_ was
+  // ever set so far
+  int max_active_threads_sofar_;
+
   std::mutex list_mu_;
   std::list<WorkerThread*> completed_threads_;
 };