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d31a976026
Summary:
When building a large Xcode project with multiple module dependencies, and mixed Objective-C & Swift, I observed a large number of clang processes stalling at zero CPU for 30+ seconds throughout the build. This was especially prevalent on my 18-core iMac Pro.
After some sampling, the major cause appears to be the lock file implementation for precompiled modules in the module cache. When the lock is heavily contended by multiple clang processes, the exponential backoff runs in lockstep, with some of the processes sleeping for 30+ seconds in order to acquire the file lock.
In the attached patch, I implemented a more aggressive polling mechanism that limits the sleep interval to a max of 500ms, and randomizes the wait time. I preserved a limited form of exponential backoff. I also updated the code to use cross-platform timing, thread sleep, and random number capabilities available in C++11.
On iMac Pro (2.3 GHz Intel Xeon W, 18 core):
Xcode 11.1 bundled clang:
502.2 seconds (average of 5 runs)
Custom clang build with LockFileManager patch applied:
276.6 seconds (average of 5 runs)
This is a 1.82x speedup for this use case.
On MacBook Pro (4 core 3.1GHz Intel i7):
Xcode 11.1 bundled clang:
539.4 seconds (average of 2 runs)
Custom clang build with LockFileManager patch applied:
509.5 seconds (average of 2 runs)
As expected, machines with fewer cores benefit less from this change.
```
Call graph:
2992 Thread_393602 DispatchQueue_1: com.apple.main-thread (serial)
2992 start (in libdyld.dylib) + 1 [0x7fff6a1683d5]
2992 main (in clang) + 297 [0x1097a1059]
2992 driver_main(int, char const**) (in clang) + 2803 [0x1097a5513]
2992 cc1_main(llvm::ArrayRef<char const*>, char const*, void*) (in clang) + 1608 [0x1097a7cc8]
2992 clang::ExecuteCompilerInvocation(clang::CompilerInstance*) (in clang) + 3299 [0x1097dace3]
2992 clang::CompilerInstance::ExecuteAction(clang::FrontendAction&) (in clang) + 509 [0x1097dcc1d]
2992 clang::FrontendAction::Execute() (in clang) + 42 [0x109818b3a]
2992 clang::ParseAST(clang::Sema&, bool, bool) (in clang) + 185 [0x10981b369]
2992 clang::Parser::ParseFirstTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&) (in clang) + 37 [0x10983e9b5]
2992 clang::Parser::ParseTopLevelDecl(clang::OpaquePtr<clang::DeclGroupRef>&) (in clang) + 141 [0x10983ecfd]
2992 clang::Parser::ParseExternalDeclaration(clang::Parser::ParsedAttributesWithRange&, clang::ParsingDeclSpec*) (in clang) + 695 [0x10983f3b7]
2992 clang::Parser::ParseObjCAtDirectives(clang::Parser::ParsedAttributesWithRange&) (in clang) + 637 [0x10a9be9bd]
2992 clang::Parser::ParseModuleImport(clang::SourceLocation) (in clang) + 170 [0x10c4841ba]
2992 clang::Parser::ParseModuleName(clang::SourceLocation, llvm::SmallVectorImpl<std::__1::pair<clang::IdentifierInfo*, clang::SourceLocation> >&, bool) (in clang) + 503 [0x10c485267]
2992 clang::Preprocessor::Lex(clang::Token&) (in clang) + 316 [0x1098285cc]
2992 clang::Preprocessor::LexAfterModuleImport(clang::Token&) (in clang) + 690 [0x10cc7af62]
2992 clang::CompilerInstance::loadModule(clang::SourceLocation, llvm::ArrayRef<std::__1::pair<clang::IdentifierInfo*, clang::SourceLocation> >, clang::Module::NameVisibilityKind, bool) (in clang) + 7989 [0x10bba6535]
2992 compileAndLoadModule(clang::CompilerInstance&, clang::SourceLocation, clang::SourceLocation, clang::Module*, llvm::StringRef) (in clang) + 296 [0x10bba8318]
2992 llvm::LockFileManager::waitForUnlock() (in clang) + 91 [0x10b6953ab]
2992 nanosleep (in libsystem_c.dylib) + 199 [0x7fff6a22c914]
2992 __semwait_signal (in libsystem_kernel.dylib) + 10 [0x7fff6a2a0f32]
```
Differential Revision: https://reviews.llvm.org/D69575
101 lines
3.1 KiB
C++
101 lines
3.1 KiB
C++
//===--- LockFileManager.h - File-level locking utility ---------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#ifndef LLVM_SUPPORT_LOCKFILEMANAGER_H
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#define LLVM_SUPPORT_LOCKFILEMANAGER_H
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#include "llvm/ADT/Optional.h"
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#include "llvm/ADT/SmallString.h"
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#include <system_error>
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#include <utility> // for std::pair
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namespace llvm {
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class StringRef;
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/// Class that manages the creation of a lock file to aid
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/// implicit coordination between different processes.
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///
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/// The implicit coordination works by creating a ".lock" file alongside
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/// the file that we're coordinating for, using the atomicity of the file
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/// system to ensure that only a single process can create that ".lock" file.
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/// When the lock file is removed, the owning process has finished the
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/// operation.
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class LockFileManager {
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public:
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/// Describes the state of a lock file.
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enum LockFileState {
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/// The lock file has been created and is owned by this instance
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/// of the object.
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LFS_Owned,
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/// The lock file already exists and is owned by some other
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/// instance.
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LFS_Shared,
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/// An error occurred while trying to create or find the lock
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/// file.
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LFS_Error
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};
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/// Describes the result of waiting for the owner to release the lock.
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enum WaitForUnlockResult {
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/// The lock was released successfully.
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Res_Success,
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/// Owner died while holding the lock.
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Res_OwnerDied,
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/// Reached timeout while waiting for the owner to release the lock.
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Res_Timeout
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};
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private:
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SmallString<128> FileName;
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SmallString<128> LockFileName;
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SmallString<128> UniqueLockFileName;
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Optional<std::pair<std::string, int> > Owner;
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std::error_code ErrorCode;
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std::string ErrorDiagMsg;
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LockFileManager(const LockFileManager &) = delete;
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LockFileManager &operator=(const LockFileManager &) = delete;
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static Optional<std::pair<std::string, int> >
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readLockFile(StringRef LockFileName);
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static bool processStillExecuting(StringRef Hostname, int PID);
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public:
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LockFileManager(StringRef FileName);
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~LockFileManager();
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/// Determine the state of the lock file.
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LockFileState getState() const;
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operator LockFileState() const { return getState(); }
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/// For a shared lock, wait until the owner releases the lock.
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/// Total timeout for the file to appear is ~1.5 minutes.
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/// \param MaxSeconds the maximum total wait time in seconds.
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WaitForUnlockResult waitForUnlock(const unsigned MaxSeconds = 90);
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/// Remove the lock file. This may delete a different lock file than
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/// the one previously read if there is a race.
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std::error_code unsafeRemoveLockFile();
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/// Get error message, or "" if there is no error.
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std::string getErrorMessage() const;
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/// Set error and error message
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void setError(const std::error_code &EC, StringRef ErrorMsg = "") {
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ErrorCode = EC;
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ErrorDiagMsg = ErrorMsg.str();
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}
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};
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} // end namespace llvm
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#endif // LLVM_SUPPORT_LOCKFILEMANAGER_H
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