May 2010 - libdispatch-dev - lists.macosforge.org

A couple of issues with the libdispatch project page
by Brent Fulgham 25 May '10

25 May '10

I wanted to get back up-to-speed with the current GCD goings on, and started reading through the libdispatch website. Unfortunately, I had a few problems with the information on the libdispatch page: 1. The git repository on the Wiki (git://git.macosforge.org/libdispatch.git) is a dead link. Is there a different repository path? 2. The wiki link for the libdispatch-changes mailing list is incorrect (instead, it points to the libdispatch-dev mailing list). The "*-changes" mailing list page is present on the server if you correct the link in your browser. I did try to correct this myself, but even as a logged in MacOSForge user it did not provide an editing option. Finally, there appear to be some significant differences in the source code released as part of the 10.6.3 Open Source release (84.5.5: http://opensource.apple.com/source/libdispatch/libdispatch-84.5.5/) compared to the contents of the svn repository. Should we assume that the repository is the most current? Thanks, -Brent

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GCD MPM for Apache
by Robert N. M. Watson 19 May '10

19 May '10

Dear all: I'm pleased to announce a new GCD-related software project, a GCD MPM for Apache. Apache Multi-Processing Modules (MPMs) allow operating systems to provide execution and concurrency structure to Apache: processes, threads, event management primitives, etc. It is intended to illustrate both GCD's programming model and programmability benefits, as well as how GCD can be integrated into a existing large-scale and thread-aware application. A quick skim of the code will bring to light some of its key benefits: a work model integrated with the GCD concurrency framework avoids complex utility libraries to distribute work and manage concurrency, and the integration of C Blocks into the programming language avoids the complexity-overhead of a message passing model. The GCD MPM is experimental and illustrative, but we hope it will also be useful in the future! The patch and instructions for applying it can be found on the Mac OS X forge web page: http://libdispatch.macosforge.org/trac/wiki/apache In the GCD MPM, connections are associated with dispatch queues, which enforce ordered processing of a connection, but allows connections to float across actual OS-provided threads via GCD. The web page says more about the approach, but one interesting observation is that the implementation is around 950 lines of code, vs. the threaded models which come in 1.6x-3x the number of lines. Thanks to Apple for their support of this project! Robert Watson University of Cambridge Computer Laboratory

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Using libdispatch/GCD in a real-time context
by Stéphane Letz 18 May '10

18 May '10

Hi, We are interested to see if libdispatch/GCD could be used in a real-time context, more specifically in JACK audio server (http://jackaudio.org/) JACK audio server can connect a number of different applications to an audio device, as well as allowing them to share audio between themselves. Its clients can run in their own processes (ie. as normal applications), or can they can run within the JACK server (ie. as a "plugin"). JACK allows synchronous execution of all clients, and low latency operation. All JACK applications are connected in a graph and the server (triggered by a CoreAudio audio callback) is responsible to "execute the graph" at each audio cycle, starting from the inputs and transferring "activation" from client to client until the outputs of the graph have been executed. On OSX JACK uses mach semaphores for inter-process communication. Each client start a thread and give it RT behavior (more precisely the thread is a "time-constraint" thread in OSX terminology and inherit the "period, computation, constraints" parameters of the server CoreAudio RT thread). On multi-cores machines, audio clients may themselves want to use additional helper threads, to speed up audio computation. This can be done using a pool of RT threads, but obviously doing like that does not scale well, when several applications do the same thing. We looked at libdispatch/GCD as a possible alternative for this "explicitly managing RT threads" model. We were thinking in completely dropping the client RT thread and replace it by a "dispatch_source" to be triggered by the inter-process semaphore. Then use libdispatch again to replace the need for RT helper threads. Right now there is no way to define a real-time queue in libdispatch, but would this kind of extension be theoreticaly possible? What would be the way to do that? Any comment on the overall idea? Best Regards Stéphane Letz

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libdispatch build on MacOS-X does not execute events
by Fabien Mottet 07 May '10

07 May '10

Hi, all. I checkouted libdispatch sources on libdispatch.macosforge.org svn rev-188. I built it under MacOS-X. Then, I try to run binaries in testing folder. They run, but created events are never executed (when I put a printf in async functions, I don't see it). By the way, when I do the build with standard MacOS-X libraries, the runs are good and I see my printf. Here is my configure adapted command from INSTALL file: ./configure \ --with-apple-libc-source=/Users/me/GCD/Libc-594.1.4 \ --with-apple-xnu-source=/Users/me/GCD/xnu-1504.3.12 \ --enable-legacy-api \ --disable-libdispatch-init-constructor \ --enable-apple-crashreporter-info \ --enable-apple-tsd-optimizations \ --enable-apple-semaphore-optimizations To have the build work, I had to: -add Block_private.h file -#define USE_APPLE_CRASHREPORTER_INFO 0 into config/config.h -Change __PTK_LIBDISPATCH_KEYX to Libc hard values since the symbols were no found at build I have MacOS-X 10.6.3 and Xcode 3.2.2. Do you have any idea how to make it work? Thank you, -- Fabien Mottet

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