Cast struct from byte buffer

Question

I am trying to read a struct from a binary byte buffer using cast and pack. I was trying to keep track of worst case read time from in memory buffer so I decided to keep a chrono high resolution clock nano timer. Whenever the timer increased I printed the value. It gave me a worst case scenario of about 20 micro seconds which was huge considering the size of the struct. When I measured the average time taken it came out to be ~20 nanoseconds. Then I measured how many times was I breaching 50. And it turns out of the ~20 million times, I was breaching 50 nanoseconds only 500 times.

My question is what can possibly cause this performance fluctuation: average of 20 and worst of 20,000?

Secondly, how can I ensure a constant time performance. I am compiling with -O3 and C++11.

 // new approach
 #pragma pack(push, 1)
 typedef struct {
    char a;
    long b, c;
    char d, name[10];
    int e , f;
    char g, h;
    int h, i;
} myStruct;
#pragma pack(pop)


//in function where i am using it


 auto am1 = chrono::high_resolution_clock::now();
 myStruct* tmp = (myStruct*)cTemp;
 tmp->name[10] = 0;
 auto am2 = chrono::high_resolution_clock::now();
 chrono::duration<long, nano> arM = chrono::duration_cast<chrono::nanoseconds>(am2 - am1);
 if(arM.count() > maxMPO.count())
 {
     cout << "myStruct read time increased: "  << arM.count() <<     "\n";
 maxMPO = arM;
 }

I am using g++4.8 with C++11 and an ubuntu server.

@Bartek, what external factors? any suggestions on how to go about it? — nashar1
– nashar1, Commented Sep 26, 2014 at 13:43
Yep, you might try using a proper benchmarking library that will repeat the tests automatically and report that information. — Bartek Banachewicz
– Bartek Banachewicz, Commented Sep 26, 2014 at 13:43
@Naveen: You mean tmp->name[9] = 0 (9 is the last valid index of an array of size 10). I think that given your use case the average time is more important than the maximum 'spike' time? — Cameron
– Cameron, Commented Sep 26, 2014 at 14:47
@Naveen: You still can't put a null byte at name[10], that's undefined behaviour and is almost certainly overwriting the low (or high if big endian) byte of e. You'll need to copy the bytes out into a larger buffer and put the null character there (obviously, only do this at the last minute when you need to actually print them out using c-strings -- otherwise, just keep them non-null terminated). — Cameron
– Cameron, Commented Sep 26, 2014 at 15:17

2785528 · Accepted Answer · 2014-09-26 14:26:05Z

1

what can possibly cause this performance fluctuation: avg of 20 and worst of 20,000?

On a PC (or Mac, or any desktop), there are Ethernet interrupts, timers, mem-refresh, and dozens of other things going on over which you have no (or very little) control.

You might consider changing the target. If you use a single board computer (SBC) with only static ram, and a network connection which you can turn off and disconnect, and timers and clocks and every other kind of interrupt under your software control, you might achieve an acceptable result.

I once worked with a gal who wrote software for an 8085 SBC. When we hooked up a scope and saw the waveform stability of a software controlled bit, I thought she must have added logic chips. It was amazing.

You simply can not achieve 'jitter' free behaviour on a desktop.

answered Sep 26, 2014 at 14:26

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1 Comment

nashar1 Over a year ago

It's not a desktop. It's an intel xeon ubuntu 14.04 server. But I guess ehternet jitters etc. would apply to it as well.

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