Re: [PATCH] aarch64: revert memcpy optimze for kunpeng to avoid performance degradation

["Zhangxuelei (Derek)" <zhangxuelei4@huawei.com>]

Hi,

They are my colleagues and we have certified this results together. It would be better to revert the original selection according to the negative performance of a specific product. And we will still study for a better or more balanced version of memcpy on Kunpeng.

Thank you~

-----邮件原件-----
发件人: Adhemerval Zanella [mailto:adhemerval.zanella@linaro.org] 
发送时间: 2021年1月20日 21:09
收件人: wangshuo (AF) <wangshuo47@huawei.com>; Zhangxuelei (Derek) <zhangxuelei4@huawei.com>; libc-alpha@sourceware.org
抄送: Hushiyuan <hushiyuan@huawei.com>; liqingqing (C) <liqingqing3@huawei.com>
主题: Re: [PATCH] aarch64: revert memcpy optimze for kunpeng to avoid performance degradation

Hi,

Since I don't have access to this specific hardware, it would be good if the original author, Xuelei Zhang, of the change could certify this reversion is ok.

It should be ok during the freeze since it just a selection of an already tested implementation for an specific chip implementation.

On 20/01/2021 04:20, Shuo Wang wrote:
> In commit 863d775c481704baaa41855fc93e5a1ca2dc6bf6, kunpeng920 is 
> added to default memcpy version, however, there is performance degradation when the copy size is some large bytes, eg: 100k.
> This is the result, tested in glibc-2.28:
>              before backport  after backport	 Performance improvement
> memcpy_1k      0.005              0.005                 0.00%
> memcpy_10k     0.032              0.029                 10.34%
> memcpy_100k    0.356              0.429                 -17.02%
> memcpy_1m      7.470              11.153                -33.02%
> 
> This is the demo
> #include "stdio.h"
> #include "string.h"
> #include "stdlib.h"
> 
> char a[1024*1024] = {12};
> char b[1024*1024] = {13};
> int main(int argc, char *argv[])
> {
>     int i = atoi(argv[1]);
>     int j;
>     int size = atoi(argv[2]);
>     
>     for (j = 0; j < i; j++)
>         memcpy(b, a, size*1024);
>     return 0;
> }
> 
> # gcc -g -O0 memcpy.c -o memcpy
> # time taskset -c 10 ./memcpy 100000 1024
> 
> Co-authored-by: liqingqing <liqingqing3@huawei.com>
> 
> ---
>  sysdeps/aarch64/multiarch/memcpy.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/sysdeps/aarch64/multiarch/memcpy.c 
> b/sysdeps/aarch64/multiarch/memcpy.c
> index 27259d3386..0e0a5cbcfb 100644
> --- a/sysdeps/aarch64/multiarch/memcpy.c
> +++ b/sysdeps/aarch64/multiarch/memcpy.c
> @@ -37,7 +37,7 @@ extern __typeof (__redirect_memcpy) __memcpy_falkor 
> attribute_hidden;  libc_ifunc (__libc_memcpy,
>              (IS_THUNDERX (midr)
>  	     ? __memcpy_thunderx
> -	     : (IS_FALKOR (midr) || IS_PHECDA (midr) || IS_KUNPENG920 (midr)
> +	     : (IS_FALKOR (midr) || IS_PHECDA (midr)
>  		? __memcpy_falkor
>  		: (IS_THUNDERX2 (midr) || IS_THUNDERX2PA (midr)
>  		  ? __memcpy_thunderx2
>