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RDTSCP versus RDTSC + CPUID


RDTSCP versus RDTSC + CPUID

By : Midori
Date : November 17 2020, 01:00 AM
hope this fix your issue A full discussion of the overhead you're seeing from the cpuid instruction is available at this stackoverflow thread. When using rdtsc, you need to use cpuid to ensure that no additional instructions are in the execution pipeline. The rdtscp instruction flushes the pipeline intrinsically. (The referenced SO thread also discusses these salient points, but I addressed them here because they're part of your question as well).
You only "need" to use cpuid+rdtsc if your processor does not support rdtscp. Otherwise, rdtscp is what you want, and will accurately give you the information you are after.
code :
uint64_t s, e;
s = rdtscp();
do_interrupt();
e = rdtscp();

atomic_add(e - s, &acc);
atomic_add(1, &counter);
   T1                              T2
t0 atomic_add(e - s, &acc);
t1                                 a = atomic_read(&acc);
t2                                 c = atomic_read(&counter);
t3 atomic_add(1, &counter);
t4                                 avg = a / c;
for (int i = 0; i < SOME_LARGEISH_NUMBER; i++) {
   s = rdtscp();
   loop_body();
   e = rdtscp();
   acc += e - s;
}

printf("%"PRIu64"\n", (acc / SOME_LARGEISH_NUMBER / CLOCK_SPEED));
s = rdtscp();
for (int i = 0; i < SOME_LARGEISH_NUMBER; i++) {
   loop_body();
}
e = rdtscp();
printf("%"PRIu64"\n", ((e-s) / SOME_LARGEISH_NUMBER / CLOCK_SPEED));


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Difference between rdtscp, rdtsc : memory and cpuid / rdtsc?

Difference between rdtscp, rdtsc : memory and cpuid / rdtsc?


By : Joel
Date : March 29 2020, 07:55 AM
wish help you to fix your issue As mentioned in a comment, there's a difference between a compiler barrier and a processor barrier. volatile and memory in the asm statement act as a compiler barrier, but the processor is still free to reorder instructions.
Processor barrier are special instructions that must be explicitly given, e.g. rdtscp, cpuid, memory fence instructions (mfence, lfence, ...) etc.
cpuid + rdtsc and out-of-order execution

cpuid + rdtsc and out-of-order execution


By : Rimantas
Date : March 29 2020, 07:55 AM
hop of those help? Since RDTSC does not depend on any input (it takes no arguments) in principle the OOO pipeline will run it as soon as it can. The reason you add a serializing instruction before it is not to let the RDTSC execute earlier.
There is an answer from John McCalpin here, you might find it useful. He explains the OOO reordering for the RDTSCP instruction (which behaves differently from RDTSC) which you may prefer to use instead.
Using rdtsc + rdtscp across context switch

Using rdtsc + rdtscp across context switch


By : H. Upchurch
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , I've done this before, and it seems to be a valid way of measuring context switch time. Whenever doing timing of something this fine-grained, scheduling unpredictability is always going to come into play; usually you deal with that by measuring thousands of times and looking for figures like the minimum, media, or mean time interval. You can make scheduling less of an issue by running both processes with real-time SCHED_FIFO priority. If you want to know actual switching time (on a single cpu core) you need to bind both processes to a single cpu with affinity settings. If you just want to know latency for one process being able to respond to the output of another, letting them run on different cpus is fine.
Another issue to keep in mind is that voluntary and involuntary context switches, and switches starting from user-space versus from kernel-space have different costs. Yours is likely to be voluntary. Measuring involuntary is harder and requires poking at shared memory from busy loops, or similar.
Why is CPUID + RDTSC unreliable?

Why is CPUID + RDTSC unreliable?


By : Ahmed EL Moselhy
Date : March 29 2020, 07:55 AM
I wish this helpful for you I think they're finding that CPUID inside the measurement interval causes extra variability in the total time. Their proposed fix in 3.2 Improvements Using RDTSCP Instruction highlights the fact that there's no CPUID inside the timed interval when they use CPUID / RDTSC to start, and RDTSCP/CPUID to stop.
Perhaps they could have ensured EAX=0 or EAX=1 before executing CPUID, to choose which CPUID leaf of data to read (http://www.sandpile.org/x86/cpuid.htm#level_0000_0000h), in case CPUID time taken depends on which query you make. Other than that, I'm unsure why that would be.
Is there any difference in between (rdtsc + lfence + rdtsc) and (rdtsc + rdtscp) in measuring execution time?

Is there any difference in between (rdtsc + lfence + rdtsc) and (rdtsc + rdtscp) in measuring execution time?


By : Tonino
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , TL;DR
rdtscp and lfence/rdtsc have the same exact upstream serialization properties On Intel processors. On AMD processors with a dispatch-serializing lfence, both sequences have also the same upstream serialization properties. With respect to later instructions, rdtsc in the lfence/rdtsc sequence may be dispatched for execution simultaneously with later instructions. This behavior may not be desirable if you also want to precisely time these later instructions as well. This is generally not a problem because the reservation station scheduler prioritizes older uops for dispatching as long as there are no structural hazards. After lfence retires, rdtsc uops would be the oldest in the RS with probably no structural hazards, so they will be immediately dispatched (possibly together with some later uops). You could also put an lfence after rdtsc.
code :
lfence                    lfence
rdtsc      -- ALLOWED --> B
B                         rdtsc

rdtscp     -- ALLOWED --> B
B                         rdtscp
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