Please create an account to participate in the Slashdot moderation system


Forgot your password?

Comment Re:a disgrace to humanity (Score 1) 199

The problem is mostly one of marketing. If you're a top tier university, students will trust that teaching you Scheme or whatever is sensible because you've got a good reputation. If you're a second tier university, then students (and parents) have heard of Java and C# and know that industry likes them, and if one university uses them as a teaching language but you don't, then it's hard to convince them that it really does make sense. And your reputation is dependent on how well your former students do, which depends on the quality of your intake.

Comment Re:Farts in their general direction. (Score 1) 445

Assuming that you can trust Dropbox, there's also the problem of compartmentalisation. My OS provides primitives that allow it to restrict which applications have access to which files. All major mobile operating systems use these primitives to do varying degrees of sandboxing. If I grant an application access to Dropbox, how do I prevent it from accessing every single file I have there? How do I audit the access that each application has? How do I ensure that this corresponds with the global policy that I have for sharing data between applications?

Comment Re:Memory is cheap (Score 2) 407

Volatile memory is defined as memory that requires power to preserve its contents. All of the 'cheap' memory that you can buy at the moment is volatile memory (and the non-volatile memory is either really expensive, really slow, or both). If you double the amount of RAM, then you double the number of capacitors that must be refreshed every few nanoseconds and so you double the power consumption. You also double the amount of heat that's generated and must be dissipated.

Now, what's the number one complaint that people have about most smartphones? Is it performance, or is it battery life? If it's performance, then doubling the RAM makes sense. If it's battery life, then doubling the RAM will make people complain more.

Comment Re:CPython uses reference counting, like objective (Score 1) 407

I implemented GC for the GNUstep Objective-C runtime using the Boehm collector (which, amusingly, performed better than Apple's 'highly optimised' collector in my tests, but then there's a reason that the AutoZone team doesn't work at Apple anymore...) and didn't notice any problems with pausing in typical applications, and did see a noticeable decrease in total memory usage, largely due to programs not leaving objects in autorelease pools for as long.

GC support was deprecated for two reasons. The first is that lots of Objective-C idioms rely on deterministic destruction. Although people are discouraged from using objects to manage scarce resources, you still have classes like NSFileHandle that own a file descriptor and close it when the object is destroyed. Nondeterminism of destruction made these problems really hard to debug.

The second problem was that the memory model for ObjC in GC mode sucked beyond belief. Or, rather, the lack of a memory model. It allowed mixing GC'd and non-GC'd buffers at will, but didn't expose this information in the type system. Consider this toy example:

void makeSomeObjects(id *buffer, int count)
for (int i=0 ; i<count ; i++)
buffer[i] = [SomeClass new];

Will this work? Who knows. If the caller passes a buffer created by NSAllocateCollectable() with NSScannedOption, or passes a buffer on the stack, then yes. If they pass the address of a global that's declared in an Objective-C source file, then yes. If they pass the address of a global that's declared in a C/C++/whatever source file, then no. If they pass a buffer created with malloc(), then no. If they pass a buffer created with NSAllocateCollectable() but without NSScannedOption, then no.

In short, the whole idea was a clusterfuck from the start. On the other hand, it may be one that's worth revisiting now that Objective-C (with ARC) has a notion of ownership for memory. Source code that uses ARC could be compiled to use GC without a huge amount of pain (although the ARC memory barrier functions seem to have been created with the express intention of not allowing GC to be added later).

Comment Re:intercept memeory allocation (Score 1) 407

Having worked in HPC, allow me to translate your post for the benefit of everyone else:

Hi, I work on a subset of problems where aliasing is intentionally reduced and where the algorithms that we use are, by design, amenable to shared-nothing parallelism, although we used shared memory for efficiency. The code that I write has very little similarity in structure or requirements to the vast majority of software in the world, but because it runs on very expensive hardware I regard my views base on this code as being valid in any setting. Memory management is easy.

Comment Re:always (Score 1) 407

You can leak memory in a garbage collected language quite easily - especially one like JavaScript that doesn't have any kind of weak reference (ECMAScript 5 draft fixes this with WeakMap, by the way), by keeping pointers around longer than you need to.

Comment Re:Easy (Score 1) 407

No, refcounts and GC are completely different.

You started well, but then ended up badly. Garbage collection is an abstract concept, reference counting is a concrete mechanism. GC means deleting objects after they are no longer referenced. It can be implemented manually, using reference counts, using tracing, or using reference counts with automatic cycle detection. The last two are equivalently expressive, but have very different cache interactions. The rest of your post talks about one specific subset of implementations of tracing garbage collectors.

Comment Re:Easy (Score 1) 407

The problem is that there isn't a good interface between the kernel and applications in most desktop operating systems (OS X is the exception, L4 HURD does it the best way but has 2 users) for letting them know that you're getting low on physical memory. The kernel knows when you're low on memory, but doesn't have any knowledge of the application's data structures so can't make very intelligent decisions about what to swap out. The application can easily throw away stuff from caches, but isn't ever told that it needs to. Mobile operating systems provide applications with explicit notifications that now would be a good time to delete some cached data. On a modern desktop, some of the trades are now very weird - it's often cheaper to refetch data from the network than it is to do a few page faults (although only if you're not on an SSD).

Comment Re:a disgrace to humanity (Score 2) 199

It's possibly related to the results. Many of the top-tier universities use less industry-friendly languages for teaching undergraduates. At Cambridge we do a lot of ocaml, at MIT they use Scheme (and, apparently, Python), at a number of others they use Haskell. There are several reasons for doing this. The first is that teaching a less common language means that you don't start the course with half the students thinking that they already know the material. The second is that teaching a relatively simple yet expressive language teaches students to think about algorithms first and then about microoptimisation later when they learn their third or forth language.

In many other universities, there has been the growing trend to believe that the language that you should teach with is one that you would use to solve real-world problems. I believe that this is a mistake, because the requirements for a language for teaching and for creating maintainable large-scale applications are nowhere near the same. This would mean, however, that students from universities with this belief would have an advantage in these contests as they'd be using a language that they'd had a few years more practice with.

Looking at the results, however, I suspect that there's also a lot of apathy involved. MIT and Stanford are there, but they tend to encourage a very competitive atmosphere. Several universities that I'd expect to produce students that would do well appear not to have entered at all. Given the wide range of extra curricular activities available to students these days, I wouldn't be surprised if entering a competition is somewhere down the list.

Slashdot Top Deals

Research is what I'm doing when I don't know what I'm doing. -- Wernher von Braun