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
to reflect the new license.
We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.
Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.
llvm-svn: 351636
On Windows we've observed that if you open a file, write to it, map it into
memory and close the file handle, the contents of the memory mapping can
sometimes be incorrect. That was what we did when adding an entry to the
ThinLTO cache using the TempFile and MemoryBuffer classes, and it was causing
intermittent build failures on Chromium's ThinLTO bots on Windows. More
details are in the associated Chromium bug (crbug.com/786127).
We can prevent this from happening by keeping a handle to the file open while
the mapping is active. So this patch changes the mapped_file_region class to
duplicate the file handle when mapping the file and close it upon unmapping it.
One gotcha is that the file handle that we keep open must not have been
created with FILE_FLAG_DELETE_ON_CLOSE, as otherwise the operating system
will prevent other processes from opening the file. We can achieve this
by avoiding the use of FILE_FLAG_DELETE_ON_CLOSE altogether. Instead,
we use SetFileInformationByHandle with FileDispositionInfo to manage the
delete-on-close bit. This lets us remove the hack that we used to use to
clear the delete-on-close bit on a file opened with FILE_FLAG_DELETE_ON_CLOSE.
A downside of using SetFileInformationByHandle/FileDispositionInfo as
opposed to FILE_FLAG_DELETE_ON_CLOSE is that it prevents us from using
CreateFile to open the file while the flag is set, even within the same
process. This doesn't seem to matter for almost every client of TempFile,
except for LockFileManager, which calls sys::fs::create_link to create a
hard link from the lock file, and in the process of doing so tries to open
the file. To prevent this change from breaking LockFileManager I changed it
to stop using TempFile by effectively reverting r318550.
Differential Revision: https://reviews.llvm.org/D48051
llvm-svn: 334630
We've been running doxygen with the autobrief option for a couple of
years now. This makes the \brief markers into our comments
redundant. Since they are a visual distraction and we don't want to
encourage more \brief markers in new code either, this patch removes
them all.
Patch produced by
for i in $(git grep -l '\\brief'); do perl -pi -e 's/\\brief //g' $i & done
Differential Revision: https://reviews.llvm.org/D46290
llvm-svn: 331272
This move some of the complexity over to the lower level TempFile.
It also makes it a bit more explicit where errors are ignored since we
now have a call to consumeError.
llvm-svn: 318550
std::error_code can represent success, so we don't need a
Optional<std::error_code>.
Rename the variable to avoid confusion with the type Error.
llvm-svn: 318111
Summary:
Previously, we would emit error messages like "IO failure on output
stream". This change causes use to include information about what
actually went wrong, e.g. "No space left on device".
Reviewers: sunfish, rnk
Reviewed By: rnk
Subscribers: mehdi_amini, llvm-commits, hiraditya
Differential Revision: https://reviews.llvm.org/D39203
llvm-svn: 316404
This is currently used by clang to lock access to modules; improve the
error message so that clang can use better output messages from locking
error issues.
rdar://problem/26529101
Differential Review: http://reviews.llvm.org/D20942
llvm-svn: 271755
Removed some unused headers, replaced some headers with forward class declarations.
Found using simple scripts like this one:
clear && ack --cpp -l '#include "llvm/ADT/IndexedMap.h"' | xargs grep -L 'IndexedMap[<]' | xargs grep -n --color=auto 'IndexedMap'
Patch by Eugene Kosov <claprix@yandex.ru>
Differential Revision: http://reviews.llvm.org/D19219
From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 266595
5 minutes is an eternity, so try to strike a better balance between
waiting long enough for any reasonable module build and not so long that
users kill the process because they think it's hanging.
Also give the client a way to delete the lock file after a timeout.
llvm-svn: 228603
Ladd Van Tol