SERVER-20944 distribute mongos work across multiple TaskExecutors

1 files changed, 113 insertions, 0 deletions

diff --git a/src/mongo/executor/task_executor_pool.h b/src/mongo/executor/task_executor_pool.h
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index 00000000000..00a828b3c27
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+++ b/src/mongo/executor/task_executor_pool.h
@@ -0,0 +1,113 @@
+/**
+ *    Copyright (C) 2015 MongoDB Inc.
+ *
+ *    This program is free software: you can redistribute it and/or  modify
+ *    it under the terms of the GNU Affero General Public License, version 3,
+ *    as published by the Free Software Foundation.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    GNU Affero General Public License for more details.
+ *
+ *    You should have received a copy of the GNU Affero General Public License
+ *    along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the GNU Affero General Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <memory>
+#include <vector>
+
+#include "mongo/platform/atomic_word.h"
+
+namespace mongo {
+namespace executor {
+
+class TaskExecutor;
+
+/**
+ * Represents a pool of TaskExecutors. Work which requires a TaskExecutor can ask for an executor
+ * from the pool. This allows for work to be distributed across several executors.
+ *
+ * The TaskExecutor and its owned NetworkInterface can become a performance bottleneck, so this
+ * pooling approach provides better scalability under concurrent query workloads.
+ *
+ * Initialization and shutdown methods are not thread-safe, but getArbitraryExecutor() may be called
+ * concurrently by multiple threads.
+ */
+class TaskExecutorPool final {
+public:
+    /**
+     * Based on properties of the system (e.g. the number of cores), suggests a number of executors
+     * to add to the pool.
+     */
+    static size_t getSuggestedPoolSize();
+
+    /**
+     * Initializes the underlying executors. This method may be called at most once for the lifetime
+     * of an executor.
+     *
+     * May only be called after populating the pool with addExecutors().
+     */
+    void startup();
+
+    /**
+     * Shuts down all underlying executors and waits for them to finish. May block.
+     *
+     * If startup() has been called, this method must eventually be called exactly once. Must be
+     * called after startup().
+     */
+    void shutdownAndJoin();
+
+    /**
+     * Adds 'executors' and 'fixedExecutor' to the pool. May be called at most once to initialize an
+     * empty pool.
+     */
+    void addExecutors(std::vector<std::unique_ptr<TaskExecutor>> executors,
+                      std::unique_ptr<TaskExecutor> fixedExecutor);
+
+    /**
+     * Returns a pointer to one of the executors in the pool. Two calls to this method may return
+     * different executors. Invalid to call if the pool has not been initialized with
+     * addExecutors().
+     *
+     * Use this method if you need a TaskExecutor for performing performance-critical work.
+     *
+     * Thread-safe.
+     */
+    TaskExecutor* getArbitraryExecutor();
+
+    /**
+     * Returns a pointer to the pool's fixed executor. Every call to this method will return the
+     * same executor. Invalid to call if the fixed executor has not been initialized with
+     * addExecutors().
+     *
+     * Use this method if you are *not* using the TaskExecutor for performance-critical work.
+     *
+     * Thread-safe.
+     */
+    TaskExecutor* getFixedExecutor();
+
+private:
+    AtomicUInt32 _counter;
+
+    std::vector<std::unique_ptr<TaskExecutor>> _executors;
+
+    std::unique_ptr<TaskExecutor> _fixedExecutor;
+};
+
+}  // namespace executor
+}  // namespace mongo