Russell Finn rsfinn

libdispatch efficiency tips

The libdispatch is one of the most misused API due to the way it was presented to us when it was introduced and for many years after that, and due to the confusing documentation and API. This page is a compilation of important things to know if you're going to use this library. Many references are available at the end of this document pointing to comments from Apple's very own libdispatch maintainer (Pierre Habouzit).

My take-aways are:

You should create very few, long-lived, well-defined queues. These queues should be seen as execution contexts in your program (gui, background work, ...) that benefit from executing in parallel. An important thing to note is that if these queues are all active at once, you will get as many threads running. In most apps, you probably do not need to create more than 3 or 4 queues.
Go serial first, and as you find performance bottle necks, measure why, and if concurrency helps, apply with care, always validating under system pressure. Reuse

	# Xcode 4.3.3
	Apple clang version 3.1 (tags/Apple/clang-318.0.61) (based on LLVM 3.1svn)
	Target: x86_64-apple-darwin11.4.0
	Thread model: posix

	# Xcode 4.3.2
	Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
	Target: x86_64-apple-darwin11.4.0
	Thread model: posix

	Application Specific Information:
	ProductBuildVersion: 4H127
	ASSERTION FAILURE in /SourceCache/IDEInterfaceBuilder/IDEInterfaceBuilder-3084/Framework/Connections/Interface/IBConnectionPopUpMenu.m:388
	Details: Need a menu.
	Function: NSMenuItem IBPopUpConnectionMenuWithMenuItems(NSArray , NSMenuItem , NSEvent , NSRect, BOOL, NSSet , CGFloat, NSWindow , NSColor , NSColor , NSColor , NSColor , id<IBConnectionPopUpMenuDelegate>)
	Thread: <NSThread: 0x40030a1e0>{name = (null), num = 1}
	Hints: None
	Backtrace:
	0 0x000000010c77b249 -[IDEAssertionHandler handleFailureInFunction:fileName:lineNumber:messageFormat:arguments:] (in IDEKit)
	1 0x000000010ba2ec65 _DVTAssertionHandler (in DVTFoundation)

	"""
	Build recursive hash of files in directory tree in hashdeep format.


	Hashdeep format description:
	http://md5deep.sourceforge.net/start-hashdeep.html

	hashdeep.py differences from original hashdeep:

	- if called without arguments, automatically starts to build

	// Taken from the commercial iOS PDF framework http://pspdfkit.com.
	// Copyright (c) 2014 Peter Steinberger, PSPDFKit GmbH. All rights reserved.
	// Licensed under MIT (http://opensource.org/licenses/MIT)
	//
	// You should only use this in debug builds. It doesn't use private API, but I wouldn't ship it.

	// PLEASE DUPE rdar://27192338 (https://openradar.appspot.com/27192338) if you would like to see this in UIKit.

	#import <objc/runtime.h>
	#import <objc/message.h>

	# coding: utf-8

	require 'rubygems'
	require 'hpricot'
	require 'fileutils'
	require 'safe_yaml'
	require 'time'

	module JekyllImport
	# This importer takes a wordpress.xml file, which can be exported from your

	/// Observes a run loop to detect any stalling or blocking that occurs.
	///
	/// This class is thread-safe.
	@interface GHRunLoopWatchdog : NSObject

	/// Initializes the receiver to watch the specified run loop, using a default
	/// stalling threshold.
	- (id)initWithRunLoop:(CFRunLoopRef)runLoop;

	/// Initializes the receiver to detect when the specified run loop blocks for

	/**
	Function that locks an object while it is being acted upon in the closure parameter.
	Rethrows allows the function to throw an error for throwing closures or not for non-throwing closures (do not need `try` for non-throwing closures).
	parameter lock: Object to be sync'd
	parameter closure: Code of critical section
	*/
	public func synchronized<T>(lock: AnyObject, @noescape closure: () throws -> T) rethrows -> T {
	objc_sync_enter(lock)
	defer {
	objc_sync_exit(lock)

	class Logging(object):

	__name__ = 'logger.info(1)'
	plist = '/System/Library/Preferences/Logging/Subsystems/'

	def __init__(__name__, plist, args, *kwargs):
	super(getLogger/, self).__init__()

	logger.info('Input parameters:\n'
	'accessibility: "{com.apple.Accessibility.plist}"\n'