![]() ![]() The feature is called “Invariant TSC” and guarantees a constant increment rate of the counter, so you should not worry about the dynamic change of frequency. ![]() This is well documented in “Intel® 64 and IA-32 Architectures Software Developer’s Manuals” Volume 3 section 17.13. As of Pentium 4 processors however the time-stamp counter increments at a constant rate. The counter counts the processor cycles which used to be a problem as processors might throttle their frequency or the counter might even stop in some circumstances (low power mode, sleep etc.). You can access it with in-line assembler in C/C++ or with a compiler intrinsic ( _rdtsc on VS). It returns a 64-bit value with the counter in the EDX:EAX registers. There is some misconception around this instruction and indeed it is kind of tricky. Modern CPUs implement a time stamp counter that starts at 0 on processor reset and steadily increases. RDTSC (Read Time-stamp counter) – Here the fun begins. The resolution is very high – in the nanosecond range.ģ. The resolution is system dependent but on modern systems my observations show that it uses the RDTSC instruction and the overhead compared to ‘raw’ RDTSC, although significant, is not that bad unless you profile very small functions. QueryPerformaceCounter() – This is the preferred way according to Microsoft as stated here. There is way to increases the resolution via timeBeginPeriod() however this also increases the pressure on the Kernel for thread scheduling and the best resolution is still too low – 1 ms.Ģ. The best frequency I achieve with GetSystemTime() is 5 milliseconds. The resolution provided by those is abysmal and they should not be used for any measurement. GetTickCount(), GetSystemTime(), timeGetTime() etc. Keep in mind that I’m looking only at ways to get timestamps for performance measurements and I’ll concentrate on Windows, although some of the techniques are not OS-specific.ġ. However people are still confused, so I’ll try to make an overview of the possible approaches and at the end of the post there is an extensive list of references. I’ll deal with context switches as a source of error for performance measurement in another post and will concentrate on just getting the timestamp right.Ī ton of articles, blog posts and threads have been written on this subject. Pushing aside context switches, interrupts and other sources of error in modern systems, this is pretty straight forward IFF you have a reliable way to get a ‘now’ timestamp. The obvious way to measure how much time (or maybe cycles) a piece of code takes is: On top of this there are the sort of instant filters and auto enhancement tools you’d expect from a (primarily) mobile photo editor, plus a cropping tool that includes a variety of aspect ratios, perspective correction, and straightening tools.When measuring the performance of some code, a critical point to get right is having an accurate and reliable method to get some kind of timestamp at exactly the moments we are interested in. You can also copy the edits from one photo and apply them to another (or multiple), and see before and after previews of any changes you make. All of these operations are non-destructive, which means you can hit the “Reset Adjustments” button at any time to get back to your original image. You can also use a set of advanced editing tools to adjust the curves of an image, view the histogram of a photo, reduce noise, and perform selective color edits. You can also use the app to edit monochrome images, touch-up blemishes with the Retouch tool, remove red-eye, adjust the white balance, add sharpening, fix or add vignetting, and more. Basic editing tools are divided into two categories: Light (including Exposure, Shadows, Highlights, Contrast, and so on) and Color (Saturation, Vibrance, and Cast).
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