Developers often say, threading is hard. Threading isn't hard, concurrency is hard. With the release of Snow Leopard Apple introduced Grand Central Dispatch, a library designed to ease the inherent difficulties in writing concurrent applications. For the non-developers reading this, you may be surprised to know that making an application do more than one thing at a time (reliably) is a difficult problem to solve as the complexity of the application increases. Computer science students are gradually introduced to the concepts of multitasking, and one of the engineering problems often used in teaching is the ‘sleeping barber problem’. The sleeping barber problem is described well on Wikipedia. It can be summarized with a few statements: The barber shop has a limited number of waiting chairs. When there are no waiting chairs, customers are turned away. The barber can only cut one customer's hair at a time. The barber should sleep when there are no customers.
We can think of the waiting chairs as a queue of customers, but we need to limit the number of items in the queue to a maximum number. GCD allows us to use one of the three global concurrent queues which execute tasks concurrently, whereas any queues that we create ourselves are serial queues. We can create a serial queue to represent our chairs, but we need to limit the number of items in the queue to a maximum number, for our purposes we'll have 3 waiting chairs in the shop. To do this we'll use a dispatch semaphore. Semaphores in GCD allow you to manage access of threads to finite resources. They have a key performance advantage over posix semaphores in that they don't call into the kernel if the resource is available, so they're fast. Semaphores allow you to initialize them with a value, which we use to represent the number of chairs in the waiting room. When a customer walks into the shop we call dispatch_semaphore_wait(), which attempts to decrement the semaphore. If the value of the semaphore is greater than zero, then a chair is free and the customer takes a seat.
If the semaphore is already at zero then all the chairs are taken, in which case dispatch_semaphore_wait() return value is non-zero and we can turn away the customer. The key to dispatch semaphores is the timeout value. Passing DISPATCH_TIME_FOREVER will cause any thread to wait until the value of the semaphore moves above zero and and a seat is available. If we did this our customers would be queueing up outside our shop prior to taking a seat in the waiting room. Instead we pass DISPATCH_TIME_NOW so we can see if a chair is immediately available by testing the return value of dispatch_semaphore_wait(). The customer sees that no space is available and walks away. Consider now that we have any number of seats taken by waiting customers. How do we get the barber to do some work? The first thought may be to use another semaphore, a binary one this time, and signal the barber that he has work to do. That's too much work. Instead we create another serial queue, and call this queue the barber.
When a customer takes a seat all we have to do is add a new task (shave and a haircut) onto the barber's list of jobs. When a barber begins his next task, we signal the semaphore that a waiting chair has become available. As the barber is a GCD queue, when there's no work available, libdispatch takes care of managing resources and putting the queue to sleep if necessary. All in all we've implemented a complete solution to the barbershop problem, without logging what's going on (and depending on your formatting) the bulk is about 18 lines of code (whooping Scala and beating even Clojure): That's all well and good, but we can go a step further. If the customers don't do anything while they're seated, we can again take advantage of the FIFO nature of GCD queues and lose the barber queue altogether. In this case we can simply enqueue a maximum of 3 work units onto our "waiting" queue, and they'll be executed serially anyway. The code then becomes: You could still create the barber queue and call dispatch_set_target_queue(waitingQueue, barberQueue) which will mean that as an item is dequeued from the waiting queue it gets executed on the barber queue, but there's little point, until we create more waiting rooms that is.
By creating more waiting rooms, we can set all their target queues to the barber and we can "funnel" work through the barber queue. Our original implementation has a key advantage though: while seated the customer can do some work which is guaranteed to complete before the "shave and a haircut" is dispatched to the barber. In our sample code, this "work" is the NSLog(@"Customer taking a seat"), but you can imagine a long calculation taking place in preparation for the haircut. Developers often say, "threading is hard". Threading isn't hard, concurrency is hard. Threading is just learning a new set of functions. What Apple have done with GCD is to give us an abstraction for threads, which also makes managing concurrency far easier. Moreso, where people have created functional programming languages like Scala and Clojure to address the concurrency problem, GCD (and blocks) is all just C. There is real power in that simplicity. We can approach problems from a functional standpoint using a procedural language, and more concisely even than I can describe it.
‘Beggar Woman’ Kim Harrison, ‘Sweeney Todd’ Alan McLoughlan and ‘Johanna’ Tracy O’Keeffe ready for the show The talented performers of the Light Opera Society of Tralee - or LOST as it is better known - brought the house down in Siamsa Tíre during the week with their take on gruesome black comedy musical Sweeney Todd. Directed by Aidan O'Carroll the Sondheim musical tells the tale Georgian-era London's famous serial killer Sweeney Todd, who dispatched his victims from his barber's chair to help neighbour Mrs Lovett make her famous meat pies. LOST's take on the 1970s musical smash concluded on Saturday after a three nights at Siamsa Tíre. The cast included Alan McLoughlin as Sweeney and Grainne O'Carroll as Mrs Lovett. They were backed by Cormac Sertutxa, Tracey O'Keeffe Caitríona Fitzmaurice, Adolfo Pirelli, Ian Dillane, Tim Landers, Noel King and Kim Harrison.VANCOUVER OPERA: THE CHILL OF SWEENEY TODDStephen Sondheim/Hugh Wheeler, SWEENEY TODD, Greer Grimsley (Sweeney Todd/ BenjaminBarker), Luretta Bybee (Mrs. Lovett), Rocco Rupulo (Anthony Hope), Caitlin Wood (Johanna), Pascal Charbonneau (Tobias Ragg), Doug McNaughton (Judge Turpin), conducted by Jonathan Darlington;
directed by Kim Collier, Queen Elizabeth Theatre, May 2, 2015 How dark is dark? Sweeney Todd is as dark as it gets. It is about vengeance, one of themost primal and savage of human urges. But it takes a moral turn, showing how vengeance can backfire and redound on its perpetrator. So, the audience thrills to tragedy. It is also about the destruction of innocence, one of the most disturbing of moral events. The marriage of Benjamin Barker (later, Sweeney Todd) and his young wife is destroyed by the dissolute Judge Turpin, who sends Barker to a penal colony, rapes his wife, and adopts and holds Barker’s daughter captive. The innocence of the daughter, Johanna, is destroyed by this theft of her childhood, her enforced confinement in a madhouse, and her later discovery of her father’s monstrous acts as a serial killer. Mrs. Lovett’s boy employee, Toby, is driven mad by learning her ghastly secret and her intent to murder him, when she has apparently loved him like a mother.
So, the audience thrills to depravity. Finally, Sweeney Todd is about extreme violence and the breaking of taboo—throat slashing and cannibalism. So, the audience thrills to Grand Guignol.And all this is sung! I suppose this should come as no surprise. Opera teaches us you can sing about anything, though singing a tender song about your lost daughter while you slash throats is taking it about as far as it can go, perhaps too far. There is something strange and unhealthy about this work. It reveals the merciless hypocrisy of society, and takes us to our darkest places—calling up pity and fear, as tragedy should, but also delectation, a smacking of the lips over the macabre. It makes for great theatre but is it great drama? As an audience member, you feel a little as though you are enjoying a hanging, but should you feel exalted or ashamed of yourself? Whatever the answer, Sweeney Todd is a ghoulish piece indeed—and a very powerful stage work. Powerful because of the tragic side of the story.
We see Todd move inexorably toward the destruction of everything he holds dear—his wife and his hope of a relationship with his daughter—for the sake of an obsession with vengeance. Mrs. Lovett, if not tragic, proves a poignant figure despite her amorality. She longs for a normal life, or as normal as she would be capable of. She wants love, which Sweeney is unable to give to her, and grows more desperate as he comes closer to his goal. Tragic irony is used to great effect, as in the number ‘Not while I’m around,’ during which Mrs. Lovett, singing a song of loving protection with Toby, decides to murder him.Also powerful because of the brilliance of the score. Sondheim is especially skilled at word setting, music and words forming a seamless dramatic conjunction. He is one of the few contemporary composers who knows how to write recitative and his numbers are excellent, making use of a diatonicism riddled with dissonance. They are also distinguished by biting irony, which puts a sting even in the sweetest of his melodies, such as the tribute to women sung by Todd and Judge Turpin.
The libretto is dazzling, with some of the cleverest word-choice and wordplay in opera, almost every twist of phrase bringing home the dark satire of the story. The most memorable number—‘A Little Priest’ (the list of pies)—is savagely funny. Vancouver Opera put imaginative effort into realizing the work on stage. Much of this effort went toward the set. The logic of the action of Sweeney Todd is a three-level stage—Todd’s upstairs barber shop, Mrs. Lovett’s ground level pie shop, and the basement, where the grisly business of the pie-making goes on and to which the bodies from Todd’s ingenious barber chair-chute make their horrific descent. The VO team decided to make this procedure visible, and so turned the pit into the basement and the main set a two-story three-dimensional rectangular frame, moveable both in terms of forward/backward and revolving motion, representing the other two levels. Since this is the opera’s most salient visual gimmick, the effort was justified and the device well used otherwise, providing multi-angled locations for the upstairs sequences, from the judge’s house to the Todd-Lovett establishment.
What of the displaced orchestra? Given that Jonathan Tunick’s distinctive orchestration makes it another character, with its sinister mutterings on the action, it seemed suitably present, and its placing upstage in the darkness underneath the rotating machine suitably disturbing. What of the updating from Victorian London to contemporary Every City? Updating seems to be de rigueur in productions these days, part of the mandate for ownership of the work. In the Director’s Notes, Kim Collier’s tells us that 2015 is fitting, given the schism between the haves and have-nots that obtains today. Nonetheless, a significant part of the cultural resonance at the core of the story’s meaning is seemingly sacrificed. While there may be a division between rich and poor in the 21st century (when hasn’t there been?), the seaminess and dark criminality, fear, smell and squalor of the streets of Victorian London are somehow integral to the fabric of Sweeney Todd. All photos by Tim Matheson
But the direction and movement overall were good. Kim Collier and Wendy Gorling had the set and characters moving in ways that made sense and enhanced the drama, with fluid interaction between ‘upstairs’ and ‘downstairs’. As for the cast, Greer Grimsley’s Sweeney Todd was well sung (heroically well sung, we might add, given the problems he was having with his voice throughout the run), though his playing of the part was stiff. In his spoken sections he relied on declamation rather than the naturalness of someone in conversation, and remained an actor, not a character. Luretta Bybee, in comparison, was much more natural, playing Mrs. Lovett as a broad comic character, realistic in singing and speaking. Playing Mrs. Lovett is always a matter of choices, and Ms. Bybee was a delight, if she lacked some of the pathos that can be found in the role. Pascal Charbonneau as Toby played the part well and winningly, providing an appropriate object for Mrs. Lovett’s tenderness, which turns out to be a sentimentality she is willing to discard when her relationship with Todd is threatened.