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How much your computer can do in a second

computers-are-fast.github.io

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Re: How much your computer can do in a second

#111

Post by andreyf »

chacham15 wrote: post #94

Be careful what conclusions you attempt to draw from examples when you arent sure what exactly is happening. These examples are actually very wrong and misleading. Take for example, the first code snippet about how many loops you can run in 1 second. The OP fails to realize that since the loop isnt producing anything which gets actually used, the compiler is free to optimize it out. You can see that thats exactly wha…

Seems clear the question assumes you're generating code that actually increments the integer (i.e. repeating a single add instruction).

An anecdote defending the question on a non-technical level: when I was a kid around 4 or 5 my dad had me race a computer counting to 1000. It was one of the more memorable parts of my young childhood!

Re: How much your computer can do in a second

#112

Post by e12e »

realo wrote: post #22

Yes, modern computers are fast. How fast? The speed of light is about 300,000 km/s. That translates to roughly 1 ns per foot (yeah, I mix up my units... I'm Canadian...) THUS, a computer with a clock speed of 2 GHz will be able to execute, on a single core/thread, about 4 (four !) single-clock instructions between the moment photons leave your screen, and the moment they arrive into your eye 2 feet (roughly) later. _…

The obvious conclusion is that by doubling your distance to the screen, you double your computers speed! ;-)

(in truth, I really like this image, though. On a similar note, I think I recently saw a number for the length of wiring in a modern cpu (ryzen review perhaps) - and it was a rather staggering number from the fractal layout of modern chips - 200 meters perhaps? In a square little more than 2 cm to a side, if that?)

Re: How much your computer can do in a second

#113

Post by [deleted] »

chacham15 wrote: post #94

Be careful what conclusions you attempt to draw from examples when you arent sure what exactly is happening. These examples are actually very wrong and misleading. Take for example, the first code snippet about how many loops you can run in 1 second. The OP fails to realize that since the loop isnt producing anything which gets actually used, the compiler is free to optimize it out. You can see that thats exactly wha…

[deleted]

Re: How much your computer can do in a second

#114

Post by [deleted] »

chacham15 wrote: post #94

Be careful what conclusions you attempt to draw from examples when you arent sure what exactly is happening. These examples are actually very wrong and misleading. Take for example, the first code snippet about how many loops you can run in 1 second. The OP fails to realize that since the loop isnt producing anything which gets actually used, the compiler is free to optimize it out. You can see that thats exactly wha…

[deleted]

Re: How much your computer can do in a second

#115

Post by danellis »

adrianratnapala wrote: post #97
Lxr wrote: post #28

Why isn't the first Python loop (that does nothing but pass) optimised away completely?

Compilers often don't optimise loops away because they feel that the user put them there for a reason -- after all they are so obvious that the user could have removed them herself. One use for such a loop is a delay -- not so common nowadays, but it used to be a mainstay of DOS based games etc. I bet that if GCC started aggressively optimising out empty loops, it would interact with some subtlety of concurrency to b…

Compilers can only optimize away loops if they know that the loop has no side effects. Compilers like GCC have to have a list of standard functions that they know are pure.

This can trip you up sometimes. For example, if you do something like use memset to zero out sensitive data when you're done using it, the computer can say "the result of memset is never used, and the reference to that memory is lost, so there's no need to actually make the call" and you're left with secrets in memory.

Re: How much your computer can do in a second

#116

Post by Klockan »

chacham15 wrote: post #94

Be careful what conclusions you attempt to draw from examples when you arent sure what exactly is happening. These examples are actually very wrong and misleading. Take for example, the first code snippet about how many loops you can run in 1 second. The OP fails to realize that since the loop isnt producing anything which gets actually used, the compiler is free to optimize it out. You can see that thats exactly wha…

> You can see that thats exactly what it does here: https://godbolt.org/g/NWa5yZ

You are wrong as well, that is what it is doing if you compile it with -O2 on that exact compiler. Try compiling with -O0 instead and it will include those instructions. Also the number is reasonable, modern cpu cores can execute around a billion loop iterations per second.

Re: How much your computer can do in a second

#117

Post by jvns »

chacham15 wrote: post #94

Be careful what conclusions you attempt to draw from examples when you arent sure what exactly is happening. These examples are actually very wrong and misleading. Take for example, the first code snippet about how many loops you can run in 1 second. The OP fails to realize that since the loop isnt producing anything which gets actually used, the compiler is free to optimize it out. You can see that thats exactly wha…

We automatically generated all the results in this quiz from the programs on the site. None of the loops were optimized out, we ran basically a binary search to figure out the maximum number of iterations you could run in a second.

Results and compiler optimizations will of course vary across computers, but they were correct on my laptop on the day that we ran them (in Sept. 2015).

If you want to reproduce this on your computer and see how the results are different, you can clone https://github.com/kamalmarhubi/one-second and run `run_benchmarks.py`

Re: How much your computer can do in a second

#118

Post by gizmo »

tedunangst wrote: post #106
gizmo wrote: post #88

Earlier quoted context omitted.

If slowness is the goal and if product managers insist on slowness, then you'd expect a lot of sleep() calls in commercial software (which has product managers) and no sleep() calls in open source software. This doesn't happen. What we see is that commercial software has better documentation, gets more long term maintenance and support, and supports more cumbersome enterprise features. Exactly what you'd expect when…

Actually, some software like facial recognition or OCR, does have sleep calls inserted because people don't believe it can be done as fast as it can be done. Thus, whenever I deposit checks at the ATM, I have to stand there and watch as some entirely unnecessary animation pretends to scan my check for ten seconds before I can have my card back.

Yes, the same applies to save buttons. If saving takes less than 200ms people are not confident the save really happened. There are also UI animations that deliberately slow things down to prevent the user from getting disoriented. This doesn't prove that product managers want software to be slow though.

Re: How much your computer can do in a second

#119

Post by dom0 »

p1esk wrote: post #77
DesiLurker wrote: post #63

Earlier quoted context omitted.

for a data parallel problem like that you could vectorize it using the SIMD unit. It never ceases to amaze me at any point how much of silicon real estate just sits idle because no one really bothered to look behind the curtain and tailor the code for a specific architecture. for reference, a recent Intel chip will have 256b wide vector unit (512 for server class chips) that can be treated as a vector of 8/16/32/64b…

I've heard that modern processors are designed with the expectation (or even a requirement) that no more than 10% of the chip will be active at once. In addition, there are power density constraints (even those 10% of the chip that can be active at once must not be concentrated in the same spot).

Nope, but processors, especially larger ones, have pretty advanced power and clock management which allows them to deal with power envelope and thermal envelope constraints. E.g. the two clocking and core voltage ladders in Xeon E5/E7 for AVX vs. non-AVX code. (Though this particular example is a bit of a sledge-hammer approach).

Re: How much your computer can do in a second

#120

Post by dom0 »

loeg wrote: post #40
supergarfield wrote: post #6

Earlier quoted context omitted.

That's very likely. Clang 8.0 collapses the loop with any optimizer setting other than -O0. But if you had to do it manually, loop unrolling and SIMD instructions (although not part of standard C) would be good bets and can probably get you an order of magnitude.

Clang 8.0?

Apple Clang version number is approximately the same as the Xcode version number, and not identical to the Clang release it's based on.
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