The libdispatch Project, (a.k.a. Grand Central Dispatch), for concurrency on multicore hardware


Grand Central Dispatch

Grand Central Dispatch (GCD or libdispatch) provides comprehensive support for concurrent code execution on multicore hardware.

libdispatch is currently available on all Darwin platforms. This project aims to make a modern version of libdispatch available on all other Swift platforms. To do this, we will implement as much of the portable subset of the API as possible, using the existing open source C implementation.

libdispatch on Darwin is a combination of logic in the xnu kernel alongside the user-space Library. The kernel has the most information available to balance workload across the entire system. As a first step, however, we believe it is useful to bring up the basic functionality of the library using user-space pthread primitives on Linux. Eventually, a Linux kernel module could be developed to support more informed thread scheduling.

Project Status

A port of libdispatch to Linux has been completed. On Linux, since Swift 3, swift-corelibs-libdispatch has been included in all Swift releases and is used by other swift-corelibs projects.

Opportunities to contribute and on-going work include:

  1. Develop a test suite for the Swift APIs of libdispatch.
  2. Enhance libdispatch as needed to support Swift language evolution and the needs of the other Core Libraries projects.

Build and Install

For detailed instructions on building and installing libdispatch, see


For detailed instructions on testing libdispatch, see

  • implement pthread_workqueue within libdispatch

    implement pthread_workqueue within libdispatch

    Provide an implementation of the pthread_workqueue functionality as a fully integrated component of libdispatch. Integration of the workqueue implementation into the dispatch code base simplifies the process of evolving the APIs between the two layers and thus prepares for future optimization and enhancements.

    Initially, the integrated pthread_workqueue is only enabled by default on Linux. Most of the internals are built on pthread APIs and thus should be fairly portable. However, Linux-specific code is used to dynamically estimate the number of runnable worker threads by reading /proc. Porting the thread pool management code to non-Linux platforms would entail providing similar functionality for those platforms (or otherwise restructuring the manager).

    opened by dgrove-oss 41
  • build: add a cmake based build system

    build: add a cmake based build system

    This is far from complete, but is sufficient to build a Linux version of libdispatch. It shows what a potential cmake based build system could look like, and if desired can be completed to build all the various flavours with cmake.

    opened by compnerd 34
  • [SR-761] dispatch_async is

    [SR-761] dispatch_async is "not very concurrent" and has a 1s overhead on Linux

    | | | |------------------|-----------------| |Previous ID | SR-761 | |Radar | None | |Original Reporter | @drewcrawford | |Type | Bug | |Status | Resolved | |Resolution | Done |

    Attachment: Download


    Linux x64

    Additional Detail from JIRA

    | | | |------------------|-----------------| |Votes | 0 | |Component/s | libdispatch | |Labels | Bug, Linux | |Assignee | frankeh (JIRA) | |Priority | Medium |

    md5: b2ed8e5cf6e4eaac3f1ede31d47b998f

    Issue Description:

    The attached program enqueues 10 blocks of work on the default queue. On Darwin, all blocks run in parallel, and the program completes in 5 seconds:

    0.0 block 0 enter
    0.0 block 7 enter
    0.0 block 1 enter
    0.0 block 3 enter
    0.0 block 2 enter
    0.0 block 4 enter
    0.0 block 5 enter
    0.0 block 6 enter
    0.0 block 8 enter
    0.0 block 9 enter
    5.0 block 1 leave
    5.0 block 6 leave
    5.0 block 9 leave
    5.0 block 5 leave
    5.0 block 3 leave
    5.0 block 0 leave
    5.0 block 8 leave
    5.0 block 4 leave
    5.0 block 2 leave
    5.0 block 7 leave
    5.0 finished

    On Linux however, the blocks enter the queue staggered 1 at a time, 1 second apart. This doubles the runtime of the program:

    0.0 block 1 enter
    0.0 block 0 enter
    1.0 block 2 enter
    2.0 block 3 enter
    3.0 block 4 enter
    4.0 block 5 enter
    5.0 block 1 leave
    5.0 block 0 leave
    5.0 block 6 enter
    5.0 block 7 enter
    5.0 block 8 enter
    6.0 block 2 leave
    6.0 block 9 enter
    7.0 block 3 leave
    8.0 block 4 leave
    9.0 block 5 leave
    10.0 block 6 leave
    10.0 block 7 leave
    10.0 block 8 leave
    11.0 block 9 leave
    11.0 finished

    In addition to that (rather large) performance regression, the fact that dispatch_async and/or the default queue has in practice a constant-time overhead of 1 second on Linux breaks a lot of my Darwin code, which expects a Darwin-like overhead for this call.

    bug libdispatch Linux 
    opened by drewcrawford 32
  • Rework of Dispatch overlay for Linux

    Rework of Dispatch overlay for Linux

    The lack of Objective-C and associated bridging when importing the libdispatch header files into Swift caused a number of problems including (a) missing ref count operations in Swift compiled code (b) SR-739, dispatch types not being upcastable to dispatch_object_t (c) SR-740, dispatch types not being upcastable to AnyObject (d) SR-737, dispatch types being imported as COpaquePointer

    This commit fixes all of these issues by injecting a complete Swift overlay layer that wraps the native libdispatch objects and native APIs.

    The C header files are now imported into a distinct CDispatch module that is not made available to Swift client code (which continues to import Dispatch). Code is added to Dispatch.swift to mirror the native CDispatch APIs up to the Swift Dispatch API and to wrap/unwrap values as needed across that boundary.

    This wrapping layer does add some minor space and time overheads, but after extensively exploring implementing the Swift object model within the C code of libdispatch (to avoid introducing this overhead), I concluded that the necessary changes to libdispatch would be fairly invasive, and therefore were not justified at this point in the development of the Swift/Linux port of libdispatch.

    opened by dgrove-oss 32
  • [SR-7039] can't use -static-stdlib (or -static-executable) with any Dispatch using app

    [SR-7039] can't use -static-stdlib (or -static-executable) with any Dispatch using app

    | | | |------------------|-----------------| |Previous ID | SR-7039 | |Radar | rdar://problem/38913393 | |Original Reporter | @weissi | |Type | Bug | |Status | Resolved | |Resolution | Done |

    Additional Detail from JIRA

    | | | |------------------|-----------------| |Votes | 6 | |Component/s | libdispatch | |Labels | Bug | |Assignee | @spevans | |Priority | Medium |

    md5: b75d9276b711df9f7b981f99923ed483

    is duplicated by:

    • SR-7085 Link error related to libdispatch when using -static-stdlib on Linux

    relates to:

    • SR-9368 libdispatch_static unit tests are disabled
    • SR-10625 tests for -static-*

    Issue Description:


    mkdir /tmp/test && cd /tmp/test && swift package init --type=executable
    cat > Sources/test/main.swift <<EOF
    import Dispatch
    let q = DispatchQueue(label: "q")
    q.sync { print("hello") }
    swift build -Xswiftc -static-stdlib


    works, just like without the -static-stdlib


    $ rm -rf .build && /usr/local/swift/swift-4.0.2-RELEASE-ubuntu16.04/usr/bin/swift build -Xswiftc -static-stdlib
    Compile Swift Module 'test' (1 sources)
    Linking ./.build/x86_64-unknown-linux/debug/test
    /usr/bin/ error: cannot find -ldispatch
    /tmp/test/Sources/test/main.swift:2: error: undefined reference to '_T08Dispatch0A5QueueCMa'
    /tmp/test/Sources/test/main.swift:2: error: undefined reference to '_T08Dispatch0A5QueueCACSS5label_AA0A3QoSV3qosAC10AttributesV10attributesAC20AutoreleaseFrequencyO011autoreleaseI0ACSg6targettcfcfA0_'
    /tmp/test/Sources/test/main.swift:2: error: undefined reference to '_T08Dispatch0A5QueueCACSS5label_AA0A3QoSV3qosAC10AttributesV10attributesAC20AutoreleaseFrequencyO011autoreleaseI0ACSg6targettcfcfA1_'
    /tmp/test/Sources/test/main.swift:2: error: undefined reference to '_T08Dispatch0A5QueueCACSS5label_AA0A3QoSV3qosAC10AttributesV10attributesAC20AutoreleaseFrequencyO011autoreleaseI0ACSg6targettcfcfA2_'
    /tmp/test/Sources/test/main.swift:2: error: undefined reference to '_T08Dispatch0A5QueueCACSS5label_AA0A3QoSV3qosAC10AttributesV10attributesAC20AutoreleaseFrequencyO011autoreleaseI0ACSg6targettcfcfA3_'
    /tmp/test/Sources/test/main.swift:2: error: undefined reference to '_T08Dispatch0A5QueueCACSS5label_AA0A3QoSV3qosAC10AttributesV10attributesAC20AutoreleaseFrequencyO011autoreleaseI0ACSg6targettcfC'
    clang: error: linker command failed with exit code 1 (use -v to see invocation)
    <unknown>:0: error: link command failed with exit code 1 (use -v to see invocation)
    error: terminated(1): /usr/local/swift/swift-4.0.2-RELEASE-ubuntu16.04/usr/bin/swift-build-tool -f /tmp/test/.build/debug.yaml main


    libdispatch.a isn't shipped even though it could be. Since this patch libdispatch.a should work but it needs to be compiled with

    ./configure --enable static=yes
    bug libdispatch 
    opened by weissi 29
  • Merge darwin/libdispatch-1121 to master

    Merge darwin/libdispatch-1121 to master

    Merge tag darwin/libdispatch-1121 on darwin/trunk.

    This includes the latest Apple source drop libdispatch-1121 (up from libdispatch-913.1.4 in #447), on top of darwin/darwin-012.

    opened by ktopley-apple 27
  • [SR-1923] Building Foundation with lib dispatch support fails with a glibc module error

    [SR-1923] Building Foundation with lib dispatch support fails with a glibc module error

    | | | |------------------|-----------------| |Previous ID | SR-1923 | |Radar | None | |Original Reporter | @phausler | |Type | Bug | |Status | Resolved | |Resolution | Done |

    Additional Detail from JIRA

    | | | |------------------|-----------------| |Votes | 1 | |Component/s | libdispatch | |Labels | Bug | |Assignee | None | |Priority | Medium |

    md5: f5f4c29f9470e2d6aab03501b79a0183

    Issue Description:

    On Ubuntu 15 with the build command of ./swift/utils/build-script -r --foundation --libdispatch
    Swift fails to compile modular components for dispatch:

    <module-includes>:1:10: note: in file included from <module-includes>:1:
    #include "dispatch.h"
    /home/phausler/Documents/Public/swift-corelibs-libdispatch/dispatch/dispatch.h:60:10: note: in file included from /home/phausler/Documents/Public/swift-corelibs-libdispatch/dispatch/dispatch.h:60:
    #include <dispatch/io.h>
    /home/phausler/Documents/Public/swift-corelibs-libdispatch/dispatch/io.h:359:2: error: declaration of 'off_t' must be imported from module 'SwiftGlibc.C.stdio' before it is required
            off_t offset,
    /usr/include/stdio.h:90:17: note: previous declaration is here
    typedef __off_t off_t;
    bug libdispatch 
    opened by phausler 25
  • Replace uses of @_silgen_name with uses of a shims header.

    Replace uses of @_silgen_name with uses of a shims header.

    Said shims header is in the main Swift repo because it's shared with the Darwin Dispatch overlay. This is the same effort as apple/swift#5854, but for corelibs.

    No intended functionality change.

    opened by jrose-apple 22
  • Android support

    Android support

    This changeset adds support for Android cross-compilation with the same restrictions applied to swift project.


    • --disable-build-tests: tests are failing to be built due to the lack of spawn. Until this is fixed, the flag will be useful to complete compilation
    • --with-android-ndk: Android NDK path to be used as a base for cross toolchain, SDK and support files.
    • --with-android-ndk-gcc-version: NDK GCC version to be used (defaults to 4.9)
    • --with-android-api-level: API version to be used for compilation
    • --enable-android: Enabled Android cross-compilation. In this implementation, as it is being done right now in swift, this is enforcing arm-linux-androideabi armeabi-v7a.


    There are a few things in this changeset that I'd like to iterate, assuming there are better or more generic solution that can be applied:

    • [x] src/shims/android includes: In this directory I created sys/sysctl.h + syscall.h to redirect headers to their locations in Android. -- update: moved to compiler conditionals instead of include overlay. commit
    • [x] src/shims/android libraries: libpthread.a + librt.a are dummy libraries because of the difference described here. To avoid this both libkqueue and libpwq should be modified to avoid asking libtool to link against them.
    • [x] and src/ This the first time I deal with autotools, so I think there should be a better way to implement this. (C|CXX|LD)FLAGS params I think will be needed explicitly declared somewhere to enable cross compilation, but maybe in AM_$0_FLAGS? I think there are some overwrites in the am files that were causing troubles, but they can be fixed if this is the right direction. Removing flags from ac_configure_args like that with sed seems like a terrible idea, maybe both libkqueue and libpwq need to be extended somehow to support this in a better way?
    • [x] and more…


    This is an example call to compile libdispatch for Android. This is similarly formatted to the call that build script in swift does to compile libdispatch.

    $ sh
    $ export SWIFT_ANDROID=<...>
    $ export NDK_PATH=<...>
    $ env \
    CC="$SWIFT_ANDROID/build/Ninja-ReleaseAssert/llvm-linux-x86_64/bin/clang" \
    CXX="$SWIFT_ANDROID/build/Ninja-ReleaseAssert/llvm-linux-x86_64/bin/clang++" \
    SWIFTC="$SWIFT_ANDROID/build/Ninja-ReleaseAssert/swift-linux-x86_64/bin/swiftc" \
    $SWIFT_ANDROID/swift-corelibs-libdispatch/configure \
    --with-swift-toolchain="$SWIFT_ANDROID/build/Ninja-ReleaseAssert/swift-linux-x86_64/" \
    --with-build-variant=release \
    --enable-android \
    --host=arm-linux-androideabi \
    --with-android-ndk=$NDK_PATH \
    --with-android-api-level=21 \
    $ make

    Side note: va_list issue mentioned in #155 is affecting the build process. There's no patch for that issues here, to avoid mixing problems that may be addressed in different ways.


    This compilation process creates the shared library with the following ELF header:

    ELF Header:
      Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00 
      Class:                             ELF32
      Data:                              2's complement, little endian
      Version:                           1 (current)
      OS/ABI:                            UNIX - System V
      ABI Version:                       0
      Type:                              DYN (Shared object file)
      Machine:                           ARM
      Version:                           0x1
      Entry point address:               0x0
      Start of program headers:          52 (bytes into file)
      Start of section headers:          1060700 (bytes into file)
      Flags:                             0x5000200, Version5 EABI, soft-float ABI
      Size of this header:               52 (bytes)
      Size of program headers:           32 (bytes)
      Number of program headers:         9
      Size of section headers:           40 (bytes)
      Number of section headers:         50
      Section header string table index: 49

    and dependencies

     0x00000001 (NEEDED)                     Shared library: []
     0x00000001 (NEEDED)                     Shared library: []
     0x00000001 (NEEDED)                     Shared library: []
     0x00000001 (NEEDED)                     Shared library: []
    opened by gonzalolarralde 22
  • [SR-6084] libdispatch missing from Trunk Development snapshots

    [SR-6084] libdispatch missing from Trunk Development snapshots

    | | | |------------------|-----------------| |Previous ID | SR-6084 | |Radar | rdar://problem/35040697 | |Original Reporter | dowobeha (JIRA User) | |Type | Bug | |Status | Resolved | |Resolution | Done |

    Additional Detail from JIRA

    | | | |------------------|-----------------| |Votes | 1 | |Component/s | libdispatch | |Labels | Bug | |Assignee | dgrove-oss (JIRA) | |Priority | Medium |

    Watchers: @shahmishal

    md5: 5685fbff17fef7e5a99d2d3c5371f970

    Issue Description:

    In all trunk development releases for Ubuntu 16.04 currently listed on the Swift downloads page, libdispatch is absent.

    dates="2017-09-22 2017-09-23 2017-09-24 2017-09-25 2017-09-26 2017-09-27 2017-09-28 2017-09-29 2017-09-30 2017-10-01 2017-10-06"
    for date in ${dates}; do
        echo "Downloading swift-DEVELOPMENT-SNAPSHOT-${date}-a-ubuntu16.04.tar.gz..."
        wget${date}-a/swift-DEVELOPMENT-SNAPSHOT-${date}-a-ubuntu16.04.tar.gz &> /dev/null
        tar xfvz swift-DEVELOPMENT-SNAPSHOT-${date}-a-ubuntu16.04.tar.gz &> /dev/null
    echo "Searching for libdispatch..."
    find swift-DEVELOPMENT-SNAPSHOT-* -name "*dispatch*"
    bug libdispatch 
    opened by swift-ci 21
  • [android] Update headers that were split in NDK 23

    [android] Update headers that were split in NDK 23

    NDK 23 split stdatomic.h up and that causes problems when invoked from C++ files, so use the new bits/stdatomic.h instead as a workaround, as seen in the test for aosp-mirror/[email protected]

    Building libdispatch for Android with the new NDK causes linkage issues in the headers, so I used this same workaround that can be seen in the C++ test for that Bionic commit that split the header up and libdispatch builds for Android again.

    I don't know if there's a better way to fix these linkage issues, so let me know if there is.

    opened by buttaface 21
  • dispatch.h incompatible with gcc / g++ 12

    dispatch.h incompatible with gcc / g++ 12

    $ echo "#include <dispatch/dispatch.h>" > foo.c
    $ gcc foo.c 
    In file included from /usr/include/dispatch/dispatch.h:32,
                     from foo.c:1:
    /usr/include/os/generic_unix_base.h:58:18: error: missing binary operator before token "("
       58 | #if __has_feature(assume_nonnull)
          |                  ^
    /usr/include/os/generic_unix_base.h:72:18: error: missing binary operator before token "("
       72 | #if __has_feature(attribute_availability_swift)
          |                  ^
    /usr/include/os/generic_unix_base.h:97:18: error: missing binary operator before token "("
       97 | #if __has_feature(objc_fixed_enum) || __has_extension(cxx_strong_enums)
          |                  ^
    opened by jcelerier 0
  • Swift Actor/Tasks concurrency on Linux - Lock contention in rand()

    Swift Actor/Tasks concurrency on Linux - Lock contention in rand()

    We have a workload pipeline which is chaining several thousand Actors to each other via AsyncStream processing pipeline. There is a multiplication affect that a single event at the start of the processing pipeline will be amplified as the event will be delivered to several Tasks processing the events concurrently. The processing time of each wakeup is currently quite small and on several microseconds range currently.

    Under Linux, what was observed when stressing this processing pipeline is that ~45% of the stacks show __DISPATCH_ROOT_QUEUE_CONTENDED_WAIT__(), which is leading to lock contention in glibc rand() - as there are ~60 threads which are created and they all contend here:

                7f193794a2db futex_wait+0x2b (inlined)
                7f193794a2db __GI___lll_lock_wait_private+0x2b (inlined)
                7f19378ff29b __random+0x6b (/usr/lib/x86_64-linux-gnu/
                7f19378ff76c rand+0xc (/usr/lib/x86_64-linux-gnu/
                7f1937bac612 __DISPATCH_ROOT_QUEUE_CONTENDED_WAIT__+0x12 (/usr/lib/swift/linux/

    This is occurring in every entrance of DISPATCH_ROOT_QUEUE_CONTENDED_WAIT(), while using macro _dispatch_contention_wait_until() which in turn uses _dispatch_contention_spins(), in here the rand() call comes in and the macro produces just these 4 values: 31, 63, 95 and 127 for how many pause/yield instructions to execute.

    The following example can reproduce the issue where ~28% of the time when sampling is spent in the code path mentioned. The example creates 5000 tasks which work between 1μs and 3μs and then sleep for random 6-10 milliseconds. The point of the test is to create the contention and to illustrate the issue with rand():

    // $ swift package init --type executable --name RandomTasks
    // $ cat  Sources/RandomTasks/main.swift &&  swift run -c release
    import Foundation
    let numberOfTasks = 5000
    let randomSleepRangeMs: ClosedRange<UInt64> = 6 ... 10
    // correlates closely to processing amount in micros
    let randomWorkRange: ClosedRange<UInt32> = 1 ... 3
    @available(macOS 10.15, *)
    func smallInfinitiveTask() async {
        let randomWork = UInt32.random(in: randomWorkRange)
        let randomSleepNs = UInt64.random(in: randomSleepRangeMs) * 1_000_000
        print("Task start; sleep: \(randomSleepNs) ns, randomWork: \(randomWork) ")
        while true {
            do {
                var x2: String = ""
                for _ in 1 ... 50 * randomWork {
                    x2 += "hi"
                // Thread.sleep(forTimeInterval: 0.001) // 1ms
                try await Task.sleep(nanoseconds: randomSleepNs)
            } catch {}
    @available(macOS 10.15, *)
    func startLotsOfTasks(_ tasks: Int) {
        for _ in 1 ... tasks {
            Task {
                await smallInfinitiveTask()
    if #available(macOS 10.15, *) {
    } else {
        // Fallback on earlier versions

    When run on Ryzen 5950X system, 18-19 HT cores are spent processing the workload. While on M1 Pro just ~4.


    opened by freef4ll 3
  • swift: remove Windows special case in `Block.swift`

    swift: remove Windows special case in `Block.swift`

    DISPATCH_ENUM will use a typed enum which creates a typedef to an elaborated type. This fails to be imported into Swift as a numeric type. Explicitly convert the numericCast'ed value from the Swift OptionSet to the dispatch_block_flags_t as a portable alternate spelling. This allows us to continue to import the enum as a typed value.

    Thanks to @beccadax for the pointer that this pattern may not work on Darwin (which it does not!) and the suggestion for the explicit type conversion!

    opened by compnerd 4
  • Fixed bool type for C89

    Fixed bool type for C89

    Before c99 doesn't have _Bool‘ andSTDC_VERSION'. Compensate.

    #ifndef __STDC_VERSION__
    #define __STDC_VERSION__ 0
    #if defined(_MSC_VER) || (__STDC_VERSION__ < 199901L)
    /* `MSVC' and `Before c99' doesn't have <stdbool.h>. Compensate. */
    typedef char bool;
    #define true (bool) 1
    #define false (bool) 0
    #include <stdbool.h>
    opened by damon-kwok 1
  • [Win32] Initialize Windows Runtime on queue threads

    [Win32] Initialize Windows Runtime on queue threads

    On Windows, all threads that want to use Windows Runtime or COM components must be initialized with a call to CoInitializeEx() or the slightly more modern RoInitialize() (same as Windows::Foundation::Initialize).

    This change initializes all libdispatch threads so it’s possible to use COM and Windows Runtime APIs.

    opened by triplef 0
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