Earl Ou has uploaded this change for review. (
https://gem5-review.googlesource.com/c/public/gem5/+/34915 )
Change subject: base,sim: implement a faster mutex for single thread case
......................................................................
base,sim: implement a faster mutex for single thread case
This change applies an atomic variable to check if we really need to
obtain a mutex, and uses a condition variable to notify.
See about 5% improvement in the simulation speed.
Change-Id: I7e165987dcb587b27fae90978b9b3fde6f5563ef
---
A src/base/fast_mutex.hh
M src/sim/eventq.cc
M src/sim/eventq.hh
3 files changed, 86 insertions(+), 3 deletions(-)
diff --git a/src/base/fast_mutex.hh b/src/base/fast_mutex.hh
new file mode 100644
index 0000000..b003072
--- /dev/null
+++ b/src/base/fast_mutex.hh
@@ -0,0 +1,82 @@
+/*
+ * Copyright 2020 Google, Inc.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met: redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer;
+ * redistributions in binary form must reproduce the above copyright
+ * notice, this list of conditions and the following disclaimer in the
+ * documentation and/or other materials provided with the distribution;
+ * neither the name of the copyright holders nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef __BASE_FAST_MUTEXT_HH__
+#define __BASE_FAST_MUTEXT_HH__
+
+#include <atomic>
+#include <condition_variable>
+#include <mutex>
+
+/*
+ * The FastMutex uses an atomic flag to check if we really need to obtain a
+ * mutex lock. For most cases without multi-threads event queues, e.g.
non-KVM
+ * simulation, this avoid the system call and speed up the simulation.
+ */
+class FastMutex {
+ private:
+ // A flag to record the current status:
+ // 0: no one has the lock
+ // 1: exactly one thread has the lock
+ // >1: more than one threads are waiting for the lock.
+ std::atomic<int> flag;
+ std::mutex m;
+ std::condition_variable cv;
+
+ bool test_and_set(int expected, int desired) {
+ return flag.compare_exchange_strong(expected, desired);
+ }
+
+ public:
+ FastMutex() : flag(0) {}
+
+ void
+ lock()
+ {
+ while (!test_and_set(0, 1)) {
+ std::unique_lock<std::mutex> ul(m);
+ if (flag++ == 0) // in case flag is set to 0 before we get m.
+ break;
+ cv.wait(ul);
+ }
+ }
+
+ void
+ unlock()
+ {
+ if (test_and_set(1, 0))
+ return;
+
+ {
+ std::lock_guard<std::mutex> g(m);
+ flag = 0;
+ }
+ cv.notify_all();
+ }
+};
+
+#endif // __BASE_FAST_MUTEXT_HH__
diff --git a/src/sim/eventq.cc b/src/sim/eventq.cc
index bc4864c..adce51e 100644
--- a/src/sim/eventq.cc
+++ b/src/sim/eventq.cc
@@ -32,6 +32,7 @@
#include <cassert>
#include <iostream>
+#include <mutex>
#include <string>
#include <unordered_map>
#include <vector>
diff --git a/src/sim/eventq.hh b/src/sim/eventq.hh
index aa54722..ecd9b78 100644
--- a/src/sim/eventq.hh
+++ b/src/sim/eventq.hh
@@ -41,10 +41,10 @@
#include <functional>
#include <iosfwd>
#include <memory>
-#include <mutex>
#include <string>
#include "base/debug.hh"
+#include "base/fast_mutex.hh"
#include "base/flags.hh"
#include "base/types.hh"
#include "debug/Event.hh"
@@ -622,7 +622,7 @@
Tick _curTick;
//! Mutex to protect async queue.
- std::mutex async_queue_mutex;
+ FastMutex async_queue_mutex;
//! List of events added by other threads to this event queue.
std::list<Event*> async_queue;
@@ -647,7 +647,7 @@
* @see EventQueue::lock()
* @see EventQueue::unlock()
*/
- std::mutex service_mutex;
+ FastMutex service_mutex;
//! Insert / remove event from the queue. Should only be called
//! by thread operating this queue.
--
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Gerrit-Project: public/gem5
Gerrit-Branch: develop
Gerrit-Change-Id: I7e165987dcb587b27fae90978b9b3fde6f5563ef
Gerrit-Change-Number: 34915
Gerrit-PatchSet: 1
Gerrit-Owner: Earl Ou <[email protected]>
Gerrit-MessageType: newchange
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