Feeds

A Deadlock Holiday

What to do now there's no Moore's Law?

Remote control for virtualized desktops

Stob Moore's Law, I need hardly remind a top-notch industry professional like you, states that as the density of silicon circuitry doubles, the probability of you not being able to find some sensibly-priced extra memory to fit your old lappy approaches 1.0.

In recent times it has become generally admitted that, if this well-known observation has not yet quite joined Elvis, it is at the very least fiddling determinedly with the fire exit door. Instead of increasingly quick processors, we are given increasingly cored processors. Whereas it used to take just one running instance of Access 2000 to bring your CPU usage meter to 100%, it now takes two, four or possibly 128.

Once upon a time, all one needed to know about multi-tasking code was how to hang a few lines of badly-written (I speak for myself) assembly language off the timer interrupt. Those were the days, my friend, we thought they'd carry on; yet here I am giving you a whistle-stop tour through the baffling mechanisms designed to help you envisage and write para-multi-threaded applications.

Let us begin with some ancient history.

Co- not Sub-

The great Donald Knuth described the coroutine, a mechanism for writing single-tasked multi-tasking that allowed any programmer to cope with any situation, merely by thinking of everything at once.

Although it has never really taken off as a popular programming construct, it has been very influential, in particular pioneering the important adjective 'lightweight' (subroutines are a lightweight specialisation of coroutines). Nowadays, of course, 'lightweight' is used as a synonym for 'good' throughout comp. sci., yet it still retains a special affinity for the parallel and pseudo-parallel arts.

Threads

A 'thread', abbreviating 'thread of execution', is a lightweight version of the heavier 'process', abbreviating 'process of execution' (as in the phrase 'during the process of execution, the program abruptly died'). Threads save all the tedious mucking about creating state and context demanded by processes, by simply enabling multiple threads to cavort together in the same address space, and with the same resources, like drowning kittens in a bucket of water. Adding cores to the processor improves the threading model by significantly increasing the amount of water available. (Peace: no actual kittens were drowned in the manufacture of that simile.)

In particular, threads suffer badly from 'race conditions'. The race of despised worker threads is made to do boring, low status, 'background' tasks. Meanwhile, the high privilege 'system' threads get to party with the hardware. It's the same the whole world over...

Three old ladies

In order to overcome race conditions, and perhaps to compensate for taking away our beloved 'goto' away from us, top Dutch comp. sci. genius Edsger Dijkstra invented the 'semaphore'. The semaphore is a data structure that allows friendly communication between the threads and tasks of all nations. There are two kinds: the counting semaphore and later, when it has been compiled, the binary semaphore.

The semaphore helped do away with race conditions, and for a while all was sweetness and synchronicity. But it soon became clear that a brand new peril had been introduced: the deadlock.

Today there are at least four well-known kinds of deadlock breeding in the wild:

  • Recursive deadlock, which bloody well happens again and again
  • Deadly mutual embrace deadlock, which is much less fun than it sounds, being a kind of inter-task stalemate
  • Death-of-process deadlock, where the process that claimed the semaphore dies intestate
  • Lady Dedlock, a plaintiff in Jarndyce v. Jarndyce

By way of countering the threat and obtaining a deadlock holiday, it was decided to invent the 'mutex' (or 'mutant' as it is known by posher, system threads).

There is a lot of confusion about the difference between mutexes and semaphores, which frankly I do not see as part of my business to clear up. Instead I will refer you to the conventional explanatory model which is, weirdly, based on the notion of the lockable lavatory (or securable brickhouse, as it is almost known by commoner, worker threads).

The scenario is this: imagine a loo in a restaurant of the dismal kind where you must humiliatingly apply to staff for the key. If there is one lavatory and one key, you have a mutex and a long, fidgety wait. If there are four toilets and four interchangeable keys, you have a counting semaphore. The important thing about a counting semaphore is: it doesn't prevent four threads entering one cubicle together.

This licentious view of synchronisation is disputed by other writers, who say that the difference with mutexes is that, before sitting down, they draw the bolt of ownership across the door.

My own view is: synchronisation primitives will never be understood until somebody goes over their metaphors with gallons of industrial-strength bleach.

Intelligent flash storage arrays

Next page: Yet more

More from The Register

next story
PEAK APPLE: iOS 8 is least popular Cupertino mobile OS in all of HUMAN HISTORY
'Nerd release' finally staggers past 50 per cent adoption
Microsoft to bake Skype into IE, without plugins
Redmond thinks the Object Real-Time Communications API for WebRTC is ready to roll
Microsoft promises Windows 10 will mean two-factor auth for all
Sneak peek at security features Redmond's baking into new OS
Mozilla: Spidermonkey ATE Apple's JavaScriptCore, THRASHED Google V8
Moz man claims the win on rivals' own benchmarks
Yes, Virginia, there IS a W3C HTML5 standard – as of now, that is
You asked for it! You begged for it! Then you gave up! And now it's HERE!
FTDI yanks chip-bricking driver from Windows Update, vows to fight on
Next driver to battle fake chips with 'non-invasive' methods
DEATH by PowerPoint: Microsoft warns of 0-day attack hidden in slides
Might put out patch in update, might chuck it out sooner
Ubuntu 14.10 tries pulling a Steve Ballmer on cloudy offerings
Oi, Windows, centOS and openSUSE – behave, we're all friends here
prev story

Whitepapers

Why cloud backup?
Combining the latest advancements in disk-based backup with secure, integrated, cloud technologies offer organizations fast and assured recovery of their critical enterprise data.
Getting started with customer-focused identity management
Learn why identity is a fundamental requirement to digital growth, and how without it there is no way to identify and engage customers in a meaningful way.
High Performance for All
While HPC is not new, it has traditionally been seen as a specialist area – is it now geared up to meet more mainstream requirements?
Top 5 reasons to deploy VMware with Tegile
Data demand and the rise of virtualization is challenging IT teams to deliver storage performance, scalability and capacity that can keep up, while maximizing efficiency.
Protecting against web application threats using SSL
SSL encryption can protect server‐to‐server communications, client devices, cloud resources, and other endpoints in order to help prevent the risk of data loss and losing customer trust.