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This started life as a reply to Re^2: Which 'Perl6'? (And where?), but it seems too important to bury it down there in a long dead thread as a reply to an author I promised to resist, and whom probably will not respond. So I'm putting it here to see what of any interest it arouses.


  1. Is concurrency appropriate? There are two basic motivations ... and 2) to speed things up. In the latter case, if the problem being tackled is really IO bound, turning to concurrency probably won't help.

    That is way too simplistic a view. If the problem is IO bound to a single, local, harddisk, and is uncacheable, then concurrency may not help.

    But change any of the four defining elements of that criteria; and it might -- even: probably will -- be helped by well written asynchronicity. Eg.

    1. If the IO data is, or can be, spread across multiple local physical drives; concurrency can speed overall throughput by overlapping requests to different units.
    2. If the disks are remote -- as in SAN, NAS, cloud etc. -- then again, overlapping requests can increase throughput by utilising buffering and waiting time for processing.
    3. If the drives aren't harddisks, but SSDs; or SSD buffered HDs; or PCI connected virtual drives; then overlapping several fast read requests with each slower write request can more fully utilise the available bandwidth and improve throughput.
    4. If the IO involved displays temporal locality of reference -- that is, if the nature of the processing is such that a subset of the data has multiple references over a short period of time, even if that subset changes over the longer term -- then suspending the IO for new references until re-references to existing cached data play out comes about naturally if fine-grained concurrency is used.

    And if some or all of the IO in your IO bound processing is to the network, or network attached devices; or the intranet; or the internet; or the cloud; -- eg. webserving; webcrawling; webscraping; collaborative datasets; email; SMS; customer facing; ....... -- then both:

    • Preventing IO from freezing your processing;
    • And allowing threads of execution who's IO has completed to continue as soon as a core is available -- ie. not also have to wait for any particular core to become available;

    Is mandatory for effective utilisation of modern hardware and networks; even for IO-bound processing.

    Only kernel(OS) threading provides the required combination of facilities. Cooperative multitasking (aka. 'green threads'; aka. Win95 tech) simply does not scale beyond the single core/single thread hardware of the last century.

  2. The Problem with Threads.

    The problem with "The Problem with Threads", is that it is just so much academic hot air divorced from the realities of the real world.

    Only mathematicians and computer scientists demand total determinacy; and throw their arms up in refusal to work if they don't get it.

    The rest of the world -- you, me, mothers and toddlers, doctors, lawyers, spacemen, dustmen, pilots, builders, shippers, movers & shakers, factory workers, engineers, tinkers, tailors, soldiers, sailors, rich & poor men, beggars and thieves; all have to live in the real -- asynchronous -- world, where shit happens.

    Deliveries are late; machines break down; people are sick; power-outs and system-downs occur; the inconvenient realities of life have to be accepted, lived with and dealt with.

    The problem is not that threading is hard; the problem is that people keep on saying that "threading is hard"; and then stopping there.

    Man is very adept at dealing with hard and complex tasks

    Imagine all places you'd never have been; all the things you'd never have done; if the once wide-spread belief that we would suffocate if we attempted to travel at over 30mph.

    Too trivial an example for you? Ok. Think about heart transplantation. Think about the problems of disconnecting and reconnecting the (fragile, living) large bore pipes supplying and removing the pumped liquid; the wires carrying electrical control signals; the small bore pipes carrying the lubricants needed to keep the pump alive and removing the waste. Now think about the complexities of doing a pump change whilst keeping the engine running; the passengers comfortable and the 'life force' intact. And all the while contending with all the other problems of compatibility; rejection; infection; compounded diagnosis.

    Circa. 5000 coronary transplants occurred last year. Mankind is good at doing difficult things.

    Asynchronicity and non-determinism are 'solved problems' in almost every other walk of life

    From multiple checkouts in supermarkets; to holding patterns in the skies above airport hubs; to off & on ramps on motorways; to holding tanks in petro-chemical plants; to waiting areas in airports and doctors and dentists surgeries; to carousels in baggage claims and production lines; distribution warehouses in supply chains; roundabouts and filter-in-turn; {Add the first 10 things that spring to your mind here! }.

    One day in the near future a non-indoctrinated mathematician is going to invent a symbol for an asynchronous queue.

    She'll give it a nice, technical sounding name like "Temporally Lax Composer", which will quickly become lost behind the cute acronym and new era of deterministic, asynchronous composability will ensue.

    And the academic world will rejoice, proclaim her a genius of our time, and no doubt award her a Nobel prize. (That'd be nice!)

    And suddenly the mathematicians will realise that a process or system of processes can be deterministic, without the requirement for every stage of the process (equation) to occur in temporal lockstep.

    'Safety' is the laudable imperative of the modern era.

    As in code-safety and thread-safety, but also every other kind of predictable, potentially preventable danger.

    Like piety, chastity & sobriety from bygone eras, it is hard to argue against; but the world is full (and getting fuller) of sexually promiscuous atheists who enjoy a drink; that hold down jobs, raise kids and perform charitable works. The world didn't fall apart with the wane of the religious, moral and sobriety campaigns of the past.

    In an ideal world, all corners would be rounded; flat surfaces 'soft-touch'; voltages would be low; gases non-toxic; hot water wouldn't scald; radiant elements wouldn't sear; microwaves would be confined to lead-lined bunkers; there'd be no naked flames; and every home would be fire-proof, flood-proof, hurricane-proof, tornado-proof, earthquake-proof, tsunami-proof and pestilence-proof.

    Meanwhile in the real-world, walk around your own house and see all the dangers that lurk for the unsupervised, uneducated, unwary, careless or stupid and ask yourself why do they persist? Practicality and economics.

    Theoreticians love theoretical problems; and eschew practical solutions.

    When considering concurrency, mathematicians love to invent ever so slightly more (theoretically) efficient solutions to the 'classical' problems.

    Eg. The Dining Philosophers. In a nutshell: how can 6 fil..Phillo.. guys eat their dinners using 5 forks without one or more of them starving. They'll combine locks and syncs, barriers and signals, mutexs and spinlocks and semaphores trying to claw back some tiny percentage of a quasilinear factor.

    Why? Buy another bloody fork; or use a spoon; or eat with your damn fingers.

    The problem is said to represent the situation where you have 6 computers that need to concurrently use the scarce resource of 5 tape machines. But that's dumb!

    Its not a resource problem but a capital expenditure problem. Buy another damn tape machine and save yourself 10 times its cost by avoiding having to code and maintain a complex solution. Better still, buy two extra tape machines; cos as sure as eggs is eggs, it'll be the year-end accounting run; or the Black Friday consumer spending peak when one of those tape machines defy the 3 sigma MTBF and break.

    Threading can be complex, but there are solutions to all of the problems all around us in the every day, unreliable, non-deterministic operations of every day modern life.

    And the simplest solution to many of them is to avoid creating problems in the first place. Don't synchronise (unless you absolutely have to). Don't lock (unless it is absolutely unavoidable). Don't share (unless avoiding doing so creates greater problems).

    But equally, don't throw the baby out with the bath water. Flames are dangerous; but oh so very useful.

  3. Futures et al are the future. There are much simpler, safer, higher level ways to do concurrency. I haven't tried Paul Evans' Futures, but they look the part.

    And therein lies the very crux of the problem. Most of those decrying threads; and those offering alternative to them; either haven't tried them -- because they read they were hard -- or did try them on the wrong problems, and/or using the wrong techniques; and without taking the time to become familiar with and understand their requirements and limitations.

    Futures neither remove the complexity nor solve the problems; they just bury them under the covers forcing everyone to rely upon the efficacy of their implementation and the competence of the implementors.

    And the people making the decisions are taking advice from those thread-shy novices with silver bullets and employing those with proven track records of being completely useless at implementing threaded solutions.

    The blind taking advice from the dumb and employing the incompetent.

  4. Perl 5 "threads" are very heavy. This sometimes introduces additional complexity.

    The "heaviness" of P5 threading is a misnomer. The threads aren't heavy; the implementation of shared memory is heavy. And that could easily be fixed. If there was any interest. If there wasn't an institutionalised prejudicial barrier preventing anyone even suggesting change to improve the threading support; much less supporting those with the knowledge and ideas to take them forward.

    They've basically stagnated for the past 8 or more years because p5p won't allow change.


With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday'
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

In reply to The problem with "The Problem with Threads" by BrowserUk

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