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Multithreading - What's it Mean to Developers?
sysadmn writes "Yet another reason not to count Sun out: Chip Multithreading. CMT, as Sun calls it, is the use of hardware to assist in the execution of multiple simultaneous tasks - even on a single processor. This excellent tutorial on Sun's Developer site explains the technology, and why throughput has become more important than absolute speed in the enterprise.
From the intro: Chip multi-threading (CMT) brings to hardware the concept of multi-threading, similar to software multi-threading. ... A CMT-enabled processor, similar to software multi-threading, executes many software threads simultaneously within a processor on cores. So in a system with CMT processors, software threads can be executed simultaneously within one processor or across many processors. Executing software threads simultaneously within a single processor increases a processor's efficiency as wait latencies are minimized. "
It means we're going to have to lean to program in parallel. We're going to have to parallelize our data processing and we're going to have to learn synchronization and locking methods.
This is nothing new. The decreasing returns and impending limits of single threaded processing has been upcoming for a long time now.
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Well, if your data conversions are independent, multithreading might be of benefit to you if you have a hyperthreading processor.
And are you sure you are maxing the processor? Surely you have to wait for disk or network, at least some of the time. If more than 10% or so (number pulled from ass but based on empirical observations) of you time is spent waiting for latent devices, you can benefit from multithreading even on a plain vanilla single CPU system with no hyperthreading.
It means "Difficult to reproduce bugs".
It worries me how many people just say "it means faster programs and doesn't take much more work". That mindset leads to lazy programmers who A - Can't optimize to save their jobs; and B - Don't actually understand what multithreading really does.
If you consider it easy, you've either just thrown great big global locks on most of your code, in which case your code doesn't actually parallelize well; or you've written what I refer to in my first sentence - Bugs that take an immense effort just to reproduce, nevermind track down and fix.
I know how to deal with them. It may seem easy at first, but it's actually very hard. Your program can run for days before a thread synchronization bug surfaces and it finally deadlocks. And since it's timing dependent, you can't reproduce it.
In principle there are rules to follow to avoid deadlocks and race conditions, but since they need to be manually enforced, there's always potential for error. At least with memory access bugs the hardware often shows you a segfault; with synchronization problems you usually don't even get that.
I've learned over the years that preemptive multithreading should be used only as a last resort, and even then, it's best to put exactly one synchronization point in the entire app. Self-contained tasks should be dispatched from that point and deliver their results back with little or no interaction with the other threads.
The worst thing you can do is randomly sprinkle a bunch of semaphores, mutexes, etc. all over your app.
Since I mostly work on J2EE stuff, I let the container take care of the threading for me. The one exception is J2EE Connector Architecture (JCA) bits that use the work manager. Even there, however, most of my work is simply putting a thin JCA layer in place between the outside world and the J2EE stack.
For me, these new chips simply mean increased performance for deployed apps, without any modification to the app code.
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As many others have already pointed out, Intel has had Hyperthreading available in Pentium 4 and Xeon CPUs for a couple of years now, which does exactly what the article is talking about.
As many others know, you know exactly nothing about what you are talking about. HT has basically two sets of registers so that during a cache miss which would cuase a bubble the chip switches to the other set so it doesn't sit idle. Suns chip on the other hand actually have multiple corses physically doing work at the same time. In fact were it not for Intel's hideously flawed NetBurst architecture the hideous hack that is HyperThreading would not provide any preformance increase at all (in fact it doesn't as much provide an increase as much as negate a decrease...). For evidence consider how many Pentium Ms have HT on them... Now I may not be fully correct but I didn't volunteer a comment; I only posted to prevent the misinformation of others. You'll find more on ArsTechnica. I'd link to the article but I can't find anything on their redesigned site.
Your CPU is not doing anything else, at least do something.
As far as threading is concerned, one of the few languages I've dealt with that makes mutexes, semaphores, etc. easy to deal with is Java. Most other languages bury the stuff too deep into the proprietary APIs to make them useful. Consider multithreading in win32. We need better programming languages before we can ever start reaping the benefits of good multithreading hardware.
Furthermore, we need to get rid of lazy programming. I'm tired of watching people write slow, lazy, inefficient (in terms of both memory space AND speed) code, and justify its existence with "it'll run fast on the new über-hyper-monkey-quadruple-bucky processors." Too many times, the problem is that you've got slow code running in every thread. If the code wasn't so damned lazy, programmers would care more about nifty new hardware. We're not even coming close to using our current hardware to capacity. I've got a 1.2GHz processor with 1024Mb of RAM, and my box chugs opening an M$ Word doc?! WTF?!
<soapbox>
Most programming in the world is very similar to the universal statu$ symbol in the U.S.A. - a big gas-guzzling SUV. It's not like Jane the Soccer Mom really needs 300hp to haul her kids and groceries around town. Similarly, we have lots of lazy code out there that doesn't do much of anything but consume resources and pollute the environment. A nifty new processor feature won't be noticed in the computing world because it won't get used anyway, just like Jane the Soccer Mom wouldn't notice 100 more horsepower. </soapbox>
I pity the foo that isn't metasyntactic