[llvm] [llvm][Support] Add ExponentialBackoff helper (PR #81206)
Michael Spencer via llvm-commits
llvm-commits at lists.llvm.org
Thu Feb 8 15:58:26 PST 2024
https://github.com/Bigcheese created https://github.com/llvm/llvm-project/pull/81206
This provides a simple way to implement exponential backoff using a do while loop.
Usage example (also see the change to LockFileManager.cpp):
```
ExponentialBackoff Backoff(10s);
do {
if (tryToDoSomething())
return ItWorked;
} while (Backoff.waitForNextAttempt());
return Timeout;
```
Abstracting this out of `LockFileManager` as the module build daemon will need it.
>From 5d4b769588f9ea978ac72619fc47e985d7eaea8e Mon Sep 17 00:00:00 2001
From: Michael Spencer <bigcheesegs at gmail.com>
Date: Thu, 8 Feb 2024 15:49:17 -0800
Subject: [PATCH] [llvm][Support] Add ExponentialBackoff helper
This provides a simple way to implement exponential backoff using a
do while loop.
---
.../include/llvm/Support/ExponentialBackoff.h | 65 +++++++++++++++++++
llvm/lib/Support/CMakeLists.txt | 1 +
llvm/lib/Support/ExponentialBackoff.cpp | 29 +++++++++
llvm/lib/Support/LockFileManager.cpp | 38 ++---------
llvm/unittests/Support/CMakeLists.txt | 1 +
.../Support/ExponentialBackoffTest.cpp | 31 +++++++++
6 files changed, 134 insertions(+), 31 deletions(-)
create mode 100644 llvm/include/llvm/Support/ExponentialBackoff.h
create mode 100644 llvm/lib/Support/ExponentialBackoff.cpp
create mode 100644 llvm/unittests/Support/ExponentialBackoffTest.cpp
diff --git a/llvm/include/llvm/Support/ExponentialBackoff.h b/llvm/include/llvm/Support/ExponentialBackoff.h
new file mode 100644
index 0000000000000..8208a748eac2a
--- /dev/null
+++ b/llvm/include/llvm/Support/ExponentialBackoff.h
@@ -0,0 +1,65 @@
+//===- llvm/Support/ExponentialBackoff.h ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines a helper class for implementing exponential backoff.
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_EXPONENTIALBACKOFF_H
+#define LLVM_EXPONENTIALBACKOFF_H
+
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/Error.h"
+#include <chrono>
+#include <random>
+
+namespace llvm {
+
+/// A class to help implement exponential backoff.
+///
+/// Example usage:
+/// \code
+/// ExponentialBackoff Backoff(10s);
+/// do {
+/// if (tryToDoSomething())
+/// return ItWorked;
+/// } while (Backoff.waitForNextAttempt());
+/// return Timeout;
+/// \endcode
+class ExponentialBackoff {
+public:
+ using duration = std::chrono::steady_clock::duration;
+ using time_point = std::chrono::steady_clock::time_point;
+
+ /// \param Timeout the maximum wall time this should run for starting when
+ /// this object is constructed.
+ /// \param MinWait the minimum amount of time `waitForNextAttempt` will sleep
+ /// for.
+ /// \param MaxWait the maximum amount of time `waitForNextAttempt` will sleep
+ /// for.
+ ExponentialBackoff(duration Timeout,
+ duration MinWait = std::chrono::milliseconds(10),
+ duration MaxWait = std::chrono::milliseconds(500))
+ : MinWait(MinWait), MaxWait(MaxWait),
+ EndTime(std::chrono::steady_clock::now() + Timeout) {}
+
+ /// Blocks while waiting for the next attempt.
+ /// \returns true if you should try again, false if the timeout has been
+ /// reached.
+ bool waitForNextAttempt();
+
+private:
+ duration MinWait;
+ duration MaxWait;
+ time_point EndTime;
+ std::random_device RandDev;
+ int64_t CurrentMultiplier = 1;
+};
+
+} // end namespace llvm
+
+#endif // LLVM_EXPONENTIALBACKOFF_H
diff --git a/llvm/lib/Support/CMakeLists.txt b/llvm/lib/Support/CMakeLists.txt
index e19223fdef4f1..1f2d82427552f 100644
--- a/llvm/lib/Support/CMakeLists.txt
+++ b/llvm/lib/Support/CMakeLists.txt
@@ -176,6 +176,7 @@ add_llvm_component_library(LLVMSupport
ELFAttributes.cpp
Error.cpp
ErrorHandling.cpp
+ ExponentialBackoff.cpp
ExtensibleRTTI.cpp
FileCollector.cpp
FileUtilities.cpp
diff --git a/llvm/lib/Support/ExponentialBackoff.cpp b/llvm/lib/Support/ExponentialBackoff.cpp
new file mode 100644
index 0000000000000..7e68cf67ad385
--- /dev/null
+++ b/llvm/lib/Support/ExponentialBackoff.cpp
@@ -0,0 +1,29 @@
+//===- llvm/Support/ExponentialBackoff.h ------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/ExponentialBackoff.h"
+#include <thread>
+
+using namespace llvm;
+
+bool ExponentialBackoff::waitForNextAttempt() {
+ auto Now = std::chrono::steady_clock::now();
+ if (Now >= EndTime)
+ return false;
+
+ duration CurMaxWait = std::min(MinWait * CurrentMultiplier, MaxWait);
+ std::uniform_int_distribution<uint64_t> Dist(MinWait.count(),
+ CurMaxWait.count());
+ // Use random_device directly instead of a PRNG as uniform_int_distribution
+ // often only takes a few samples anyway.
+ duration WaitDuration = std::min(duration(Dist(RandDev)), EndTime - Now);
+ if (CurMaxWait < MaxWait)
+ CurrentMultiplier *= 2;
+ std::this_thread::sleep_for(WaitDuration);
+ return true;
+}
diff --git a/llvm/lib/Support/LockFileManager.cpp b/llvm/lib/Support/LockFileManager.cpp
index a2b0fe8ca8f2e..34c7a16b24be4 100644
--- a/llvm/lib/Support/LockFileManager.cpp
+++ b/llvm/lib/Support/LockFileManager.cpp
@@ -11,6 +11,7 @@
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/Errc.h"
#include "llvm/Support/ErrorOr.h"
+#include "llvm/Support/ExponentialBackoff.h"
#include "llvm/Support/FileSystem.h"
#include "llvm/Support/MemoryBuffer.h"
#include "llvm/Support/Process.h"
@@ -20,7 +21,6 @@
#include <chrono>
#include <ctime>
#include <memory>
-#include <random>
#include <sys/stat.h>
#include <sys/types.h>
#include <system_error>
@@ -295,29 +295,15 @@ LockFileManager::waitForUnlock(const unsigned MaxSeconds) {
return Res_Success;
// Since we don't yet have an event-based method to wait for the lock file,
- // implement randomized exponential backoff, similar to Ethernet collision
+ // use randomized exponential backoff, similar to Ethernet collision
// algorithm. This improves performance on machines with high core counts
// when the file lock is heavily contended by multiple clang processes
- const unsigned long MinWaitDurationMS = 10;
- const unsigned long MaxWaitMultiplier = 50; // 500ms max wait
- unsigned long WaitMultiplier = 1;
- unsigned long ElapsedTimeSeconds = 0;
+ using namespace std::chrono_literals;
+ ExponentialBackoff Backoff(std::chrono::seconds(MaxSeconds), 10ms, 500ms);
- std::random_device Device;
- std::default_random_engine Engine(Device());
-
- auto StartTime = std::chrono::steady_clock::now();
-
- do {
+ // Wait first as this is only called when the lock is known to be held.
+ while (Backoff.waitForNextAttempt()) {
// FIXME: implement event-based waiting
-
- // Sleep for the designated interval, to allow the owning process time to
- // finish up and remove the lock file.
- std::uniform_int_distribution<unsigned long> Distribution(1,
- WaitMultiplier);
- unsigned long WaitDurationMS = MinWaitDurationMS * Distribution(Engine);
- std::this_thread::sleep_for(std::chrono::milliseconds(WaitDurationMS));
-
if (sys::fs::access(LockFileName.c_str(), sys::fs::AccessMode::Exist) ==
errc::no_such_file_or_directory) {
// If the original file wasn't created, somone thought the lock was dead.
@@ -329,17 +315,7 @@ LockFileManager::waitForUnlock(const unsigned MaxSeconds) {
// If the process owning the lock died without cleaning up, just bail out.
if (!processStillExecuting((*Owner).first, (*Owner).second))
return Res_OwnerDied;
-
- WaitMultiplier *= 2;
- if (WaitMultiplier > MaxWaitMultiplier) {
- WaitMultiplier = MaxWaitMultiplier;
- }
-
- ElapsedTimeSeconds = std::chrono::duration_cast<std::chrono::seconds>(
- std::chrono::steady_clock::now() - StartTime)
- .count();
-
- } while (ElapsedTimeSeconds < MaxSeconds);
+ }
// Give up.
return Res_Timeout;
diff --git a/llvm/unittests/Support/CMakeLists.txt b/llvm/unittests/Support/CMakeLists.txt
index df35a7b7f3626..15a126279125c 100644
--- a/llvm/unittests/Support/CMakeLists.txt
+++ b/llvm/unittests/Support/CMakeLists.txt
@@ -38,6 +38,7 @@ add_llvm_unittest(SupportTests
ErrnoTest.cpp
ErrorOrTest.cpp
ErrorTest.cpp
+ ExponentialBackoffTest.cpp
ExtensibleRTTITest.cpp
FileCollectorTest.cpp
FileOutputBufferTest.cpp
diff --git a/llvm/unittests/Support/ExponentialBackoffTest.cpp b/llvm/unittests/Support/ExponentialBackoffTest.cpp
new file mode 100644
index 0000000000000..327c86c5442f2
--- /dev/null
+++ b/llvm/unittests/Support/ExponentialBackoffTest.cpp
@@ -0,0 +1,31 @@
+//===- unittests/ExponentialBackoffTest.cpp -------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Support/ExponentialBackoff.h"
+#include "gtest/gtest.h"
+#include <chrono>
+
+using namespace llvm;
+using namespace std::chrono_literals;
+
+namespace {
+
+TEST(ExponentialBackoffTest, Timeout) {
+ auto Start = std::chrono::steady_clock::now();
+ // Use short enough times that this test runs quickly.
+ ExponentialBackoff Backoff(100ms, 1ms, 10ms);
+ do {
+ } while(Backoff.waitForNextAttempt());
+ auto Duration = std::chrono::steady_clock::now() - Start;
+ EXPECT_GE(Duration, 100ms);
+}
+
+// Testing individual wait duration is omitted as those tests would be
+// non-deterministic.
+
+} // end anonymous namespace
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