rdtsc

问题 I am running this test on a cpu with constant_tsc and nonstop_tsc $ grep -m 1 ^flags /proc/cpuinfo | sed 's/ /\n/g' | egrep "constant_tsc|nonstop_tsc" constant_tsc nonstop_tsc Step 1: Calculate the tick rate of the tsc: I calculate _ticks_per_ns as the median over a number of observations. I use rdtscp to ensure in-order execution. static const int trials = 13; std::array<double, trials> rates; for (int i = 0; i < trials; ++i) { timespec beg_ts, end_ts; uint64_t beg_tsc, end_tsc; clock

问题 This question already has answers here : How to get the CPU cycle count in x86_64 from C++? (4 answers) Closed 6 months ago . I'm new to C++ and benchmarking I don't understand what the this part of the code does? So I found something about the edx, eax registers, but I don't fully understand how that plays into the code. So I understand this code essentially returns the current tick of the cpu cycle. So, does it store the current tick into the registers, one part in the hi and the other part

问题 I have a piece of code running on MSVC 2012: #include <windows.h> #include <intrin.h> UINT64 gettime() { try { unsigned int ui; return __rdtscp(&ui); } catch (...) { return __rdtsc(); } } I was trying to use __rdtscp() to get the timestamp; however, on the platform where the __rdtscp() is not supported, I want to switch to __rdtsc() instead. The above code doesn't work; the program simply crashed if the __rdtscp() is not supported (on certain VMs). So is there any way I can detect if the _

问题 I don't know what exactly does this code: int rdtsc(){ __asm__ __volatile__("rdtsc"); Please, someone can explain me? why "rdtsc"? 回答1: Actually, that's not very good code at all. RDTSC is the x86 instruction "ReaD TimeStamp Counter" - it reads a 64-bit counter that counts up at every clock cycle of your processor. But since it's a 64-bit number, it's stored in EAX (low part) and EDX (high part), and if this code is ever used in a case where it is inlined, the compiler doesn't know that EDX

问题 I've read that RDTSC can gives false readings and should not be relied upon. Is this true and if so what can be done about it? 回答1: Very old CPU's have a RDTSC that is accurate. The problem However newer CPU's have a problem. Engineers decided that RDTSC would be great for telling time. However if a CPU throttles the frequency RDTSC is useless for telling time. The aforementioned braindead engineers then decided to 'fix' this problem by having the TSC always run at the same frequency, even if

问题 Given an x86 with a constant TSC, which is useful for measuring real time, how can one convert between the "units" of TSC reference cycles and normal human real-time units like nanoseconds using the TSC calibration factor calculated by Linux at boot-time? That is, one can certainly calculate the TSC frequency in user-land by taking TSC and clock measurements (e.g., with CLOCK_MONOTONIC ) at both ends of some interval to determine the TSC frequency, but Linux has already made this calculation

问题 Sometimes I encounter code that reads TSC with rdtsc instruction, but calls cpuid right before. Why is calling cpuid necessary? I realize it may have something to do with different cores having TSC values, but what exactly happens when you call those two instructions in sequence? 回答1: It's to prevent out-of-order execution. From a link that has now disappeared from the web (but which was fortuitously copied here before it disappeared), this text is from an article entitled "Performance

问题 As a follow-up to this topic, in order to calculate the memory miss latency, I have wrote the following code using _mm_clflush , __rdtsc and _mm_lfence (which is based on the code from this question/answer). As you can see in the code, I first load the array into the cache. Then I flush one element and therefore the cache line is evicted from all cache levels. I put _mm_lfence in order to preserve the order during -O3 . Next, I used time stamp counter to calculate the latency or reading array

问题 I'm looking into measuring benchmark performance using the time-stamp register (TSR) found in x86 CPUs. It's a useful register, since it measures in a monotonic unit of time which is immune to the clock speed changing. Very cool. Here is an Intel document showing asm snippets for reliably benchmarking using the TSR, including using cpuid for pipeline synchronisation. See page 16: http://www.intel.com/content/www/us/en/embedded/training/ia-32-ia-64-benchmark-code-execution-paper.html To read

问题 Suppose all the cores in my CPU have same frequency, technically I can synchronize system time and time stamp counter pairs for each core every millisecond or so. Then based on the current core I'm running with, I can take the current rdtsc value and using the tick delta divided by the core frequency I'm able to estimate the time passed since I last synchronized the system time and time stamp counter pair and to deduce the current system time without the overhead of system call from my