Cache vectors to avoid heap allocations on each message pump iteration.

mojo/common/message_pump_mojo.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "mojo/common/message_pump_mojo.h"
 
 #include <algorithm>
 #include <vector>
 
 #include "base/debug/alias.h"
@@ skipping 35 matching lines @@
   RunState() : should_quit(false) {
     CreateMessagePipe(NULL, &read_handle, &write_handle);
   }
 
   base::TimeTicks delayed_work_time;
 
   // Used to wake up WaitForWork().
   ScopedMessagePipeHandle read_handle;
   ScopedMessagePipeHandle write_handle;
 
+  // Cached structures to avoid the heap allocation cost of std::vector<>.
+  scoped_ptr<WaitState> wait_state;
+  scoped_ptr<HandleToHandlerList> cloned_handlers;
+
   bool should_quit;
 };
 
 MessagePumpMojo::MessagePumpMojo() : run_state_(NULL), next_handler_id_(0) {
   DCHECK(!current())
       << "There is already a MessagePumpMojo instance on this thread.";
   g_tls_current_pump.Pointer()->Set(this);
 }
 
 MessagePumpMojo::~MessagePumpMojo() {
@@ skipping 99 matching lines @@
       continue;
 
     more_work_is_plausible = delegate->DoIdleWork();
     if (run_state->should_quit)
       break;
   }
 }
 
 bool MessagePumpMojo::DoInternalWork(const RunState& run_state, bool block) {
   const MojoDeadline deadline = block ? GetDeadlineForWait(run_state) : 0;
-  const WaitState wait_state = GetWaitState(run_state);
+  if (!run_state_->wait_state)
+    run_state_->wait_state.reset(new WaitState);
+  GetWaitState(run_state, run_state_->wait_state.get());
 
   const WaitManyResult wait_many_result =
-      WaitMany(wait_state.handles, wait_state.wait_signals, deadline, nullptr);
+      WaitMany(run_state_->wait_state->handles,
+               run_state_->wait_state->wait_signals, deadline, nullptr);
   const MojoResult result = wait_many_result.result;
   bool did_work = true;
   if (result == MOJO_RESULT_OK) {
     if (wait_many_result.index == 0) {
       // Control pipe was written to.
       ReadMessageRaw(run_state.read_handle.get(), NULL, NULL, NULL, NULL,
                      MOJO_READ_MESSAGE_FLAG_MAY_DISCARD);
     } else {
-      DCHECK(handlers_.find(wait_state.handles[wait_many_result.index]) !=
+      DCHECK(handlers_.find(
+                 run_state_->wait_state->handles[wait_many_result.index]) !=
              handlers_.end());
       WillSignalHandler();
-      handlers_[wait_state.handles[wait_many_result.index]]
-          .handler->OnHandleReady(wait_state.handles[wait_many_result.index]);
+      handlers_[run_state_->wait_state->handles[wait_many_result.index]]
+          .handler->OnHandleReady(
+              run_state_->wait_state->handles[wait_many_result.index]);
       DidSignalHandler();
     }
   } else {
     switch (result) {
       case MOJO_RESULT_CANCELLED:
       case MOJO_RESULT_FAILED_PRECONDITION:
-        RemoveInvalidHandle(wait_state, result, wait_many_result.index);
+        RemoveInvalidHandle(*run_state_->wait_state, result,
+                            wait_many_result.index);
         break;
       case MOJO_RESULT_DEADLINE_EXCEEDED:
         did_work = false;
         break;
       default:
         base::debug::Alias(&result);
         // Unexpected result is likely fatal, crash so we can determine cause.
         CHECK(false);
     }
   }
+  // To keep memory usage under control, delete the WaitState object at the end
+  // if it's vectors are too big by a factor of 2. Pre-C++11 doesn't have a way
+  // to shrink vectors, so just get rid of them and re-create on the next round.
+  if (run_state_->wait_state->handles.capacity() >
+          2 * run_state_->wait_state->handles.size() ||
+      run_state_->wait_state->wait_signals.capacity() >
+          2 * run_state_->wait_state->wait_signals.size()) {

[qsr, 2015/05/28 09:33:48]

Not sure the || is needed here. The 2 vectors should follow the same pattern, or
do we expect some randomization on vector about when it does its resizing?

[Anand Mistry (off Chromium), 2015/05/28 23:27:45]

Good point. They should be in sync.

+    run_state_->wait_state.reset();
+  }
 
   // Notify and remove any handlers whose time has expired. Make a copy in case
   // someone tries to add/remove new handlers from notification.
-  const HandleToHandler cloned_handlers(handlers_);
+  if (!run_state_->cloned_handlers) {
+    run_state_->cloned_handlers.reset(new HandleToHandlerList);
+    run_state_->cloned_handlers->reserve(handlers_.size());

[qsr, 2015/05/28 09:33:48]

Isn't the reserve always useful, whether the list is new or not?

[Anand Mistry (off Chromium), 2015/05/28 23:27:45]

Done.

+  } else {
+    run_state_->cloned_handlers->clear();
+  }
+  for (const auto& handler : handlers_) {
+    run_state_->cloned_handlers->push_back(handler);
+  }
   const base::TimeTicks now(internal::NowTicks());
-  for (HandleToHandler::const_iterator i = cloned_handlers.begin();
-       i != cloned_handlers.end(); ++i) {
+  for (HandleToHandlerList::const_iterator i =
+           run_state_->cloned_handlers->begin();
+       i != run_state_->cloned_handlers->end(); ++i) {
     // Since we're iterating over a clone of the handlers, verify the handler is
     // still valid before notifying.
     if (!i->second.deadline.is_null() && i->second.deadline < now &&
         handlers_.find(i->first) != handlers_.end() &&
         handlers_[i->first].id == i->second.id) {
       WillSignalHandler();
       i->second.handler->OnHandleError(i->first, MOJO_RESULT_DEADLINE_EXCEEDED);
       DidSignalHandler();
       handlers_.erase(i->first);
       did_work = true;
     }
   }
+  if (run_state_->cloned_handlers->capacity() >
+      2 * run_state_->cloned_handlers->size()) {
+    run_state_->cloned_handlers.reset();
+  }
   return did_work;
 }
 
 void MessagePumpMojo::RemoveInvalidHandle(const WaitState& wait_state,
                                           MojoResult result,
                                           uint32_t index) {
   // TODO(sky): deal with control pipe going bad.
   CHECK(result == MOJO_RESULT_FAILED_PRECONDITION ||
         result == MOJO_RESULT_CANCELLED);
   CHECK_NE(index, 0u);  // Indicates the control pipe went bad.
@@ skipping 11 matching lines @@
 
 void MessagePumpMojo::SignalControlPipe(const RunState& run_state) {
   const MojoResult result =
       WriteMessageRaw(run_state.write_handle.get(), NULL, 0, NULL, 0,
                       MOJO_WRITE_MESSAGE_FLAG_NONE);
   // If we can't write we likely won't wake up the thread and there is a strong
   // chance we'll deadlock.
   CHECK_EQ(MOJO_RESULT_OK, result);
 }
 
-MessagePumpMojo::WaitState MessagePumpMojo::GetWaitState(
-    const RunState& run_state) const {
-  WaitState wait_state;
-  wait_state.handles.push_back(run_state.read_handle.get());
-  wait_state.wait_signals.push_back(MOJO_HANDLE_SIGNAL_READABLE);
+void MessagePumpMojo::GetWaitState(
+    const RunState& run_state,
+    MessagePumpMojo::WaitState* wait_state) const {
+  wait_state->handles.clear();
+  wait_state->wait_signals.clear();

[qsr, 2015/05/28 09:33:48]

Do you want to use resize here?

[Anand Mistry (off Chromium), 2015/05/28 23:27:45]

Nah. That would change the logic here and I don't think it's worth the
insignificant performance gain.

[qsr, 2015/05/29 07:56:08]

Sorry, I meant reserve, not resize. That will not change the logic and ensure
you won't have any allocation while doing push_back.

[Anand Mistry (off Chromium), 2015/05/29 09:00:57]

Done.

+  wait_state->handles.push_back(run_state.read_handle.get());
+  wait_state->wait_signals.push_back(MOJO_HANDLE_SIGNAL_READABLE);
 
   for (HandleToHandler::const_iterator i = handlers_.begin();
        i != handlers_.end(); ++i) {
-    wait_state.handles.push_back(i->first);
-    wait_state.wait_signals.push_back(i->second.wait_signals);
+    wait_state->handles.push_back(i->first);
+    wait_state->wait_signals.push_back(i->second.wait_signals);
   }
-  return wait_state;
 }
 
 MojoDeadline MessagePumpMojo::GetDeadlineForWait(
     const RunState& run_state) const {
   const base::TimeTicks now(internal::NowTicks());
   MojoDeadline deadline = TimeTicksToMojoDeadline(run_state.delayed_work_time,
                                                   now);
   for (HandleToHandler::const_iterator i = handlers_.begin();
        i != handlers_.end(); ++i) {
     deadline = std::min(
         TimeTicksToMojoDeadline(i->second.deadline, now), deadline);
   }
   return deadline;
 }
 
 void MessagePumpMojo::WillSignalHandler() {
   FOR_EACH_OBSERVER(Observer, observers_, WillSignalHandler());
 }
 
 void MessagePumpMojo::DidSignalHandler() {
   FOR_EACH_OBSERVER(Observer, observers_, DidSignalHandler());
 }
 
 }  // namespace common
 }  // namespace mojo