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Codel.cpp
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Codel.cpp
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/*
* Copyright 2017-present Facebook, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
#include <wangle/concurrent/Codel.h>
#include <algorithm>
#include <folly/portability/GFlags.h>
DEFINE_int32(codel_interval, 100,
"Codel default interval time in ms");
DEFINE_int32(codel_target_delay, 5,
"Target codel queueing delay in ms");
using std::chrono::nanoseconds;
using std::chrono::milliseconds;
namespace wangle {
Codel::Codel()
: codelMinDelay_(0),
codelIntervalTime_(std::chrono::steady_clock::now()),
codelResetDelay_(true),
overloaded_(false) {}
bool Codel::overloaded(std::chrono::nanoseconds delay) {
bool ret = false;
auto now = std::chrono::steady_clock::now();
// Avoid another thread updating the value at the same time we are using it
// to calculate the overloaded state
auto minDelay = codelMinDelay_;
if (now > codelIntervalTime_ &&
// testing before exchanging is more cacheline-friendly
(!codelResetDelay_.load(std::memory_order_acquire)
&& !codelResetDelay_.exchange(true))) {
codelIntervalTime_ = now + getInterval();
if (minDelay > getTargetDelay()) {
overloaded_ = true;
} else {
overloaded_ = false;
}
}
// Care must be taken that only a single thread resets codelMinDelay_,
// and that it happens after the interval reset above
if (codelResetDelay_.load(std::memory_order_acquire) &&
codelResetDelay_.exchange(false)) {
codelMinDelay_ = delay;
// More than one request must come in during an interval before codel
// starts dropping requests
return false;
} else if(delay < codelMinDelay_) {
codelMinDelay_ = delay;
}
// Here is where we apply different logic than codel proper. Instead of
// adapting the interval until the next drop, we slough off requests with
// queueing delay > 2*target_delay while in the overloaded regime. This
// empirically works better for our services than the codel approach of
// increasingly often dropping packets.
if (overloaded_ && delay > getSloughTimeout()) {
ret = true;
}
return ret;
}
int Codel::getLoad() {
// it might be better to use the average delay instead of minDelay, but we'd
// have to track it. aspiring bootcamper?
return std::min<int>(100, 100 * getMinDelay() / getSloughTimeout());
}
nanoseconds Codel::getMinDelay() {
return codelMinDelay_;
}
milliseconds Codel::getInterval() {
return milliseconds(FLAGS_codel_interval);
}
milliseconds Codel::getTargetDelay() {
return milliseconds(FLAGS_codel_target_delay);
}
milliseconds Codel::getSloughTimeout() {
return getTargetDelay() * 2;
}
} // namespace wangle