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Index > Windows > How get nanoseconds from QueryPerformanceCounter?

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Roman



Joined: 21 Apr 2012
Posts: 1088
Roman
I found this.

Code:
LARGE_INTEGER freq;
QueryPerformanceFrequency(&freq);
double nanoseconds_per_count = 1.0e9 / static_cast<double>(freq);

LARGE_INTEGER time1, time2;
QueryPerformanceCounter(&time1);
function_to_be_timed();
QueryPerformanceCounter(&time2);

int nanoseconds = (time2-time1)*nanoseconds_per_count;
    

This is correct code?
Post 11 Aug 2021, 08:12
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18445
Location: In your JS exploiting you and your system
revolution
Sure, the code looks about right, and will probably work fine.

But you won't get actual nano-second timing. Most systems have a much lower frequency counter that isn't anywhere near 1GHz. Each counter step is more than 1ns.

I suggest you use the QueryPerformanceFrequency function and display result to see what frequency your system has.
Post 11 Aug 2021, 08:19
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Roman



Joined: 21 Apr 2012
Posts: 1088
Roman
I get my Cpu ryzen 3500 frequency 10 000 000
I Wonder Why 10000000?
Why such a perfect number.

Why not 10603246?
Post 12 Aug 2021, 04:03
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18445
Location: In your JS exploiting you and your system
revolution
That sounds about right. 10MHz is quite normal for most systems. So the smallest possible resolution is 100ns.

I don't know the significance of 10603246, but you can equally ask why not <any random value>, and the answer is because that is what the system has for its timer. Somewhere on the mobo is a 10MHz clock signal feeding into a counter.
Post 12 Aug 2021, 04:46
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Roman



Joined: 21 Apr 2012
Posts: 1088
Roman
This mean 10 000 000 not 100% speed my Cpu.
Post 12 Aug 2021, 05:41
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18445
Location: In your JS exploiting you and your system
revolution
If you want the CPU clock then there is RDTSC.
Post 12 Aug 2021, 07:37
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Roman



Joined: 21 Apr 2012
Posts: 1088
Roman
Very funny.
I try two code and get the same time(almost 1 millisecond).
Code:
mov  rcx,2_042_000_000
            .i:                
                dec  rcx      
                jnz  .i
    

Code:
mov  rcx,2_042_000_000
            .i:
                inc  eax      ;!* mean do parallel
                inc  r8       ;!*
                inc  r9       ;!*
                inc  edx      ;!*
                inc  ebx      ;!*
                inc  edi      ;!*
                dec  rcx      ;!*
                jnz  .i
    

I read AMD doing 4 instructions on tact . Ryzen 3500 (Zen2). 4 Ghz
But this code show 8 instruction on tact. I'm lost .

If i write in code couple regs. Like as inc esi and inc r10 i get different time.
Post 14 Aug 2021, 05:50
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revolution
When all else fails, read the source


Joined: 24 Aug 2004
Posts: 18445
Location: In your JS exploiting you and your system
revolution
How many cycles were used to execute the loop? Use RDTSC to get the cycle count.

If it takes 4 cycles per loop, then the second loop only needs to execute 2 IPC to get the same timing.
Post 14 Aug 2021, 06:00
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