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Closed 11 years ago.
I have an interview coming up in a week's time for an entry level position that involves programming in CUDA (hopefully with C).
I was wondering if anybody can suggest some interview questions that I can expect during the interview.
I have gone through the official programming guide but I'm not all that convenient right now.
Thanks.
Some questions I think you should prepare are:
How many different kind of memories are in a GPU ?
What means coalesced / uncoalesced?
Can you implement a matrix transpose kernel?
What is a warp ?
How many warps can run simultaneously inside a multiprocessor?
What is the difference between a block and a thread ?
Can thread communicate between them? and blocks ?
Can you describe how works a cache?
What is the difference between shared memory and registers?
Which algorithms perform better on the gpu? data bound or cpu bound?
Which steps will you perform to port of an application to cuda ?
What is a barrier ?
What is a Stream ?
Can you describe what means occupancy of a kernel?
What means structure of array vs array of structures?
"You have N vectors of length M (N>>M). Tell me how you would go about designing a kernel to evaluate the distance matrix. Pay special attention to the way the problem is sub-divided and to the way the thread co-operation can be used to improve occupancy.
How would your answer to this question change if M>>N?"
The idea here is not to get you writing code, but to get you thinking out loud. This shows that you really know how to use GPGPU technology and are not merely regurgitating the user guide.
If it's a scientific role then expect questions on floating point and numerical accuracy, in particular you should look at the reduction sample in the NVIDIA SDK since that illustrates a whole load of the points in Fabrizio's post too.
Related
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Closed 9 years ago.
I came across this article: Should you ever use Linked List. It cites that given the technological advances in available memory and RAM structures, using arrays would be better than Linked List.
There's also an old question When to use a linked list over an array/array list?
Do the arguments in the article really hold and are/have Linked List become obsolete or what would be the scenarios where using a LinkedList would still be better than Arrays if the arguments are true ?
(Explainations for any point with example would be helpful)
Nonsense. O(n) will never beat constant-time. Any use of lists that's required to perform well for insertions with saved iterators will use linked lists. They're a fundamental structure and won't go away.
I'd spin the argument the other way: linked lists are more acceptable these days. On a 386, you have to be careful with performance, but now, we write programs in Python even and put up with their speed. From the amount of code written in languages that use a VM (or are interpreted) I think it's fair to say a lot of people aren't at the level of worrying about cache misses in their choice of data structure.
We have fast CPUs now, so often don't need to worry about the few extra instructions that might be needed in implementing our data structures. We can look at the uses we have, work out what requirements we have and pick our structures based on their asymptotic performance. This also makes the code more maintainable: you won't have to change in your code if you find out in six months' time that for n=100 list is quicker after all. Profiling is hard work, so we should be very comfortable in our CPU-guzzling days to pick the structure with the algorithmic properties we want rather than guessing at vector.
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Closed 10 years ago.
I tried googling it, but no solid answer. What are available, still maintained dynamic array implementations for C? What are pros and cons for every one of them, and what is the best one(speed/footprint ratio)? Just asking, so that I don't have to reinvent the wheel.
GArray from GLib does what you want.
If you are looking for something like NSMutableArray, from Objective-C, or something like ArrayList from Java, you won't find anything (std C, at least).
You can create your own dynamic array implementation in C, though. It will take you a few code lines and is not that hard to implement.
All you need to have in mind is Time vs Memory. You can do an implementation that allocates a new array with a bigger size, every time you push/add an element, and then pops it for you in the return or by reference, or you can reallocate memory every time. I don't see big advantages in neither one, except that realloc is a C library function that I think is low level implemented, meaning it is probably faster, and in matters of implementation I would go with the realloc one since it is faster to implement.
You can even build an api that gives you sorting types and clean all methods.
Now is up to you.
Hope this helps.
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Closed 9 years ago.
A professor of mine has said he'll accept homework assignments in any language we'd care to use. I'm on good enough terms that I'd like to mess with him a bit and submit a valid homework assignment using brainfuck, whitespace, or some equally "useful" language.
I have the C-sources for a few simple numerical analysis routines as well as the compiled output and the assembly files they generate.
Does anyone know of a decompiler (or a C->brainfuck translator) that could give me something akin to the "brainfuck source code"?
Just use APL or J.
Unlike BF, they were actually designed to serve a "useful" (and not a "useful as in BF" sense) purpose - and yet can easily make Perl code-golf entries look like novels. (The dedication and mental training to enjoy these languages is currently more than my skill/effort levels.)
If the goal is using a purely esoteric language, I have always enjoyed the look of Piet programs. It looks prettier and is actually able to solve common CS homework problems. Following the links will reveal "Piet assemblers" and other tools. Win.
Happy coding.
For what it's worth, I just wrote a very simple Brainfuck Assembler (inspired by this SO post actually), which assembles readable source code (not C, just something simple and nameless) to BrainFuck. The source-code and compilation/usage instructions can be found here: BrainFuck Assembler.
Edit: The project has recently been updated under a new name: BrainFix.
Edit 2: I redid the entire project. The new and improved version has quite a lot of features and is available on Github.
A quick Google search brings up the (a?) Brainfuck site, which links to an archive with "all things Brainfuck". I doubt there's a C->brainfuck translator anywhere, I wouldn't think anyone would invest that much time.
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Closed 10 years ago.
I'm looking for interesting programming puzzles, problems or challenges suitable for a class of chemistry majors learning C as their first programming language. Do you have any recommendations?
Project Euler is pretty good. They have some simple challenges that may be suitable.
These won't really do much to teach them C, though. Text books are much better for that.
Additionally, you could have them write a program to balance chemical reaction equations. That would be good for I/O and simple math.
Given a text file with a whole bunch of pressure/temperature/mole count as input, and using the ideal gas law equation, output the values of the volume for the gases and output the entire set of data (P,V,T and n) into a nicely formatted output file.
Should cover file i/o, basic function usage, and string formatting. Has the potential to cover arrays and stucts as well.
David, tasks that come to my mind would be:
calculation of the number/topology of isomers of hydrocarbons (cyclic and acyclic, saturated and unsturated)
numerical integration of optical spectra (absorption and fluorescence)
kinetical models
deconvolution of experimental data
modelling of thermodynamical cycles and their efficiencies
This seems a really vague question but assuming I'm a chemistry student that is learning C I would like to write programs that allows me to define molecules and compounds starting from simple elements.
I really don't know how to explain it, but maybe define your struct for a nitrogen atom, one for oxigen atom and have a way to bind it to produce water.. or maybe mixing to different substances too see what will come out programmatically..
You can try pex4fun. It allows you to learn algorithms in C# (which is close enough to C). pex4fun provides read-to-use classes and also engaging coding duels that turn learning into a game.
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Closed 12 years ago.
So, I'm wondering if there's a good marker to choose in C, apart from the common 0xDEADBEEFor the less appealing (for proper code) 0x0BADA550.
What's your favorite?
Is there any reason to choose one particular value or another?
Wikipedia has a whole page on this subject; it provides a lot of examples and famous software in which they are used.
Anyway, if you are working on x86 you should consider following #torak's suggestion, memory filled with int 3 saved me several times. If you feel creative, you may make it more recognizable and use CC90CC90, which translates to alternated int 3 and nop.
The only marker that I can think of that might have more than asthetic value is 0xCCCCCCCC. If, through some kind of error, it was executed 0xCC translates to an INT 3 instruction.
This will work in IA-32 and x86-64. Im not sure if there are equivalents for other architectures.
Well I always picked 0x80000000, 0x80000001 incrementing for each new region type to tickle unexpected values with signed integers. Use of these values will be strangely large for unsigned types, largely negative for signed types, and abruptly become positive for any subtractions made (thereby testing for other bugs at the same time). This has another neat side effect, in that various ALU bits will be set such as overflow, which can be detected through use of -ftrapv and other compiler flags.
0xBABECAFE, 0xBADADD00, 0xBADBAD00, 0xFADEFADE
0xCAFEBABE is, I understand, the magic number for Java. The only thing that makes one better than another is how unlikely it is to appear in data or uninitialized memory. So don't use 0x00000000 or all Fs.
It depends what you're trying to mark. For example, do you need to distinguish between allocated but uninitialized memory and deallocated memory? If so, you need different markers for the two.
Everyone has their preference. If everyone on the team uses a different marker, it can help to determine the source of an error. As in:
"DEADBEEF? Oh, this must be from John's code."