Re: x86-64: strlen-evex performance performance degradation compared to strlen-avx2

[Sunil Pandey <skpgkp2@gmail.com>]

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On Fri, Apr 26, 2024 at 6:30 AM H.J. Lu <hjl.tools@gmail.com> wrote:

> On Thu, Apr 25, 2024 at 9:03 PM abush wang <abushwangs@gmail.com> wrote:
> >
> > Hi, H.J.
> > When I test glibc performance between 2.28 and 2.38,
> > I found there is a performance degradation about strlen.
> > In fact, this difference comes from __strlen_avx2 and __strlen_evex
> >
> > ```
> > 2.28
> > __strlen_avx2 () at ../sysdeps/x86_64/multiarch/strlen-avx2.S:42
> > 42 ENTRY (STRLEN)
> >
> >
> > 2.38
> > __strlen_evex () at ../sysdeps/x86_64/multiarch/strlen-evex.S:79
> > 79 ENTRY_P2ALIGN (STRLEN, 6)
> > ```
> >
> > This is my test:
> > ```
> > #include <stdio.h>
> > #include <stdlib.h>
> > #include <stdint.h>
> > #include <string.h>
> >
> > #define MAX_STRINGS 100
> >
> > uint64_t rdtsc() {
> >     uint32_t lo, hi;
> >     __asm__ __volatile__ (
> >         "rdtsc" : "=a"(lo), "=d"(hi)
> >     );
> >     return ((uint64_t)hi << 32) | lo;
> > }
> >
> > int main(int argc, char *argv[]) {
> >     char *input_str[MAX_STRINGS];
> >     size_t lengths[MAX_STRINGS];
> >     int num_strings = 0; // Number of input strings
> >     uint64_t start_cycles, end_cycles;
> >
> >     // Parse command line arguments and store pointers in input_str array
> >     for (int i = 1; i < argc && num_strings < MAX_STRINGS; ++i) {
> >         input_str[num_strings] = argv[i];
> >         num_strings++;
> >     }
> >
> >     // Measure the strlen operation for each string
> >     start_cycles = rdtsc();
> >     for (int i = 0; i < num_strings; ++i) {
> >         lengths[i] = strlen(input_str[i]);
> >     }
> >     end_cycles = rdtsc();
> >
> >     unsigned long long total_cycle = end_cycles - start_cycles;
> >     unsigned long long av_cycle = total_cycle / num_strings;
> >     // Print the total cycles taken for the strlen operations
> >     printf("Total cycles: %llu av cycle: %llu \n", total_cycle,
> av_cycle);
> >
> >     // Print the recorded lengths
> >     printf("Lengths of the input strings:\n");
> >     for (int i = 0; i < num_strings; ++i) {
> >         printf("String %d length: %zu\n", i, lengths[i]);
> >     }
> >
> >     return 0;
> > }
> > ```
> >
> > This is result
> > ```
> > 2.28
> > ./strlen_test str1 str2 str3 str4 str5
> > Total cycles: 1468 av cycle: 293
> > Lengths of the input strings:
> > String 0 length: 4
> > String 1 length: 4
> > String 2 length: 4
> > String 3 length: 4
> > String 4 length: 4
> >
> > 2.38
> > ./strlen_test str1 str2 str3 str4 str5
> > Total cycles: 1814 av cycle: 362
> > Lengths of the input strings:
> > String 0 length: 4
> > String 1 length: 4
> > String 2 length: 4
> > String 3 length: 4
> > String 4 length: 4
> > ```
> >
> > Thanks,
> > abush
>

I'm not sure how you are measuring the performance of strlen function.
Are you making performance conclusion based on these 2 runs?

2.28
Total cycles: 1468 av cycle: 293

2.38
Total cycles: 1814 av cycle: 362

Please use glibc microbenchmark to see if you can reproduce perf drop.


>
> Which processors did you use?  Sunil, Noah, can we reproduce it?
>
> --
> H.J.
>

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