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Date: Sat, 11 Feb 2023 13:12:23 +0800 (CST)
From: "David Wang" <00107082@....com>
To: "Rich Felker" <dalias@...c.org>
Cc: musl@...ts.openwall.com, "Markus Wichmann" <nullplan@....net>
Subject: Re:Re: Re:Re: Re:Re: Re:Re: qsort
At 2023-02-10 22:19:55, "Rich Felker" <dalias@...c.org> wrote:
>On Fri, Feb 10, 2023 at 09:45:12PM +0800, David Wang wrote:
>>
>>
>>
>
>> About wrapper_cmp, in my last profiling, there are total 931387
>> samples collected, 257403 samples contain callchain ->wrapper_cmp,
>> among those 257403 samples, 167410 samples contain callchain
>> ->wrapper_cmp->mycmp, that is why I think there is extra overhead
>> about wrapper_cmp. Maybe compiler optimization would change the
>> result, and I will make further checks.
>
>Yes. On i386 here, -O0 takes wrapper_cmp from 1 instruction to 10
>instructions.
>
>Rich
With optimized binary code, it is very hard to collect an intact callchain from kernel via perf_event_open:PERF_SAMPLE_CALLCHAIN.
But to profile qsort, a callchain may not be necessary. IP register sampling would be enough to identify which part take most cpu cycles.
So I change the strategy, instead of PERF_SAMPLE_CALLCHAIN, now I just use PERF_SAMPLE_IP
This is what I got:
+-------------------+---------------+
| func | count |
+-------------------+---------------+
| Total | 423488 |
| memcpy | 48.76% 206496 |
| sift | 16.29% 68989 |
| mycmp | 14.57% 61714 |
| trinkle | 8.90% 37690 |
| cycle | 5.45% 23061 |
| shr | 2.19% 9293 |
| __qsort_r | 1.77% 7505 |
| main | 1.04% 4391 |
| shl | 0.55% 2325 |
| wrapper_cmp | 0.42% 1779 |
| rand | 0.05% 229 |
| __set_thread_area | 0.00% 16 |
+-------------------+---------------+
(Note that, in this profile report, I count only those samples that are directly within the function body, the samples within sub-function do not contribute to any of its parent functions.)
And you're right, with optimization, the impact of wrapper_cmp is very very low, only 0.42%.
The memcpy stands out above, I use uprobe(perf_event_open:PERF_SAMPLE_REGS_USER) to collect statistics about the size (the 3rd parameter, stored in RDX register) of memcpy, and all of those memcpy function calls are just copying 4 bytes, according to the source code, the size of memcpy is item size to be sorted, which is int32 in my test case.
Maybe something could be improved here.
I also made same profiling against glibc:
+-----------------------------+--------------+
| func | count |
+-----------------------------+--------------+
| Total | 640880 |
| msort_with_tmp.part.0 | 73.99 474176 | <--- merge sort?
| mycmp | 11.76 75392 |
| main | 6.45 41306 |
| __memcpy_avx_unaligned_erms | 4.58 29339 |
| random | 0.86 5525 |
| __memcpy_avx_unaligned | 0.83 5293 |
| random_r | 0.76 4882 |
| rand | 0.45 2897 |
| _init | 0.31 1975 |
| _fini | 0.01 80 |
| __free | 0.00 5 |
| _int_malloc | 0.00 5 |
| malloc | 0.00 2 |
| __qsort_r | 0.00 1 |
| _int_free | 0.00 1 |
+-----------------------------+--------------+
Test code:
-------------------
#include <stdio.h>
#include <stdlib.h>
int mycmp(const void *a, const void *b) { return *(const int *)a-*(const int*)b; }
#define MAXN 1<<20
int vs[MAXN];
int main() {
int i, j, k, n, t;
for (k=0; k<1024; k++) {
for (i=0; i<MAXN; i++) vs[i]=i;
for (n=MAXN; n>1; n--) {
i=n-1; j=rand()%n;
if (i!=j) { t=vs[i]; vs[i]=vs[j]; vs[j]=t; }
}
qsort(vs, MAXN, sizeof(vs[0]), mycmp);
}
return 0;
}
-------------------
gcc test.c -O2 -static
With musl-libc:
$ time ./a.out
real 9m 5.10s
user 9m 5.09s
sys 0m 0.00s
With glic:
$ time ./a.out
real 1m56.287s
user 1m56.270s
sys 0m0.004s
To sum up, optimize those memcpy calls and reduce comparation to its minimum could have significant performance improvements, but I doubt it could achieve a 4-factor improvement.
FYI
David
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