Cache coherence. When you scan horizontally, your data will be closer together in memory, so you will have less cache misses and thus performance will be faster. For a small enough rectangle, this won’t matter.
With a Little Bit of Help From the JVM… WARNING: This solution is now obsolete in newer Java SE versions. See other ad-hoc solutions further below. If you use an HotSpot JVM, since Java 6 update 21, you can use this command-line option: -XX:+UseCompressedStrings The JVM Options page reads: Use a byte[] for Strings which … Read more
The problem occurs because you access a char-array through a double*: char data[8]; … return *(double*)data; But gcc assumes that your program will never access variables though pointers of different type. This assumption is called strict-aliasing and allows the compiler to make some optimizations: If the compiler knows that your *(double*) can in no way … Read more
Interestingly, using -fdata-sections can make the literal pools of your functions, and thus your functions themselves larger. I’ve noticed this on ARM in particular, but it’s likely to be true elsewhere. The binary I was testing only grew by a quarter of a percent, but it did grow. Looking at the disassembly of the changed … Read more
It’s significant, especially as fragmentation grows and the allocator has to hunt harder across larger heaps for the contiguous regions you request. Most performance-sensitive applications typically write their own fixed-size block allocators (eg, they ask the OS for memory 16MB at a time and then parcel it out in fixed blocks of 4kb, 16kb, etc) … Read more
String.split(String) won’t create regexp if your pattern is only one character long. When splitting by single character, it will use specialized code which is pretty efficient. StringTokenizer is not much faster in this particular case. This was introduced in OpenJDK7/OracleJDK7. Here’s a bug report and a commit. I’ve made a simple benchmark here. $ java … Read more
Actually, deepcopy is very slow. But we can use json, ujson, or cPickle. we can use json/cPickle to dump an object, and load it later. This is my test: Total time: 3.46068 s File: test_deepcopy.py Function: test at line 15 Line # Hits Time Per Hit % Time Line Contents ============================================================== 15 @profile 16 def … Read more
You are at a disadvantage. Windows Explorer almost certainly uses FindFirstFile/FindNextFile to both traverse the directory structure and collect size information (through lpFindFileData) in one pass, making what is essentially a single system call per file. Python is unfortunately not your friend in this case. Thus, os.walk first calls os.listdir (which internally calls FindFirstFile/FindNextFile) any … Read more
Is there any faster way of doing this in Java? Yes. Scanner is fairly slow (at least according to my experience). If you don’t need to validate the input, I suggest you just wrap the stream in a BufferedInputStream and use something like String.split / Integer.parseInt. A small comparison: Reading 17 megabytes (4233600 numbers) using … Read more
Yes, under the as-if rule the compiler is only obligated to emulate the observable behavior of the code, so if you have a loop that does not have any observable behavior then it can be optimized away completely and therefore will effectively have zero execution time. Examples For example the following code: int main() { … Read more