Re: [Qemu-devel] [PATCH] net: optimize checksum computation

[Dmitry Fleytman]

> On 6 Jan 2017, at 10:08 AM, Ladi Prosek <address@hidden> wrote:
> 
> Very simple loop optimization with a significant performance impact.
> 
> Microbenchmark results, modern x86-64:
> 
> buffer size | speed up
> ------------+---------
> 1500        | 1.7x
> 64          | 1.5x
> 8           | 1.15x
> 
> Microbenchmark results, POWER7:
> 
> buffer size | speed up
> ------------+---------
> 1500        | 5x
> 64          | 3.3x
> 8           | 1.13x
> 
> There is a lot of room for further improvement at the expense of
> code complexity - aligned multibyte reads, LE/BE considerations,
> architecture-specific optimizations, etc. This patch still keeps
> things simple and readable.

Reviewed-by: Dmitry Fleytman <address@hidden>

> 
> Signed-off-by: Ladi Prosek <address@hidden>
> ---
> net/checksum.c | 21 +++++++++++++--------
> 1 file changed, 13 insertions(+), 8 deletions(-)
> 
> diff --git a/net/checksum.c b/net/checksum.c
> index 23323b0..4da72a6 100644
> --- a/net/checksum.c
> +++ b/net/checksum.c
> @@ -22,17 +22,22 @@
> 
> uint32_t net_checksum_add_cont(int len, uint8_t *buf, int seq)
> {
> -    uint32_t sum = 0;
> +    uint32_t sum1 = 0, sum2 = 0;
>     int i;
> 
> -    for (i = seq; i < seq + len; i++) {
> -        if (i & 1) {
> -            sum += (uint32_t)buf[i - seq];
> -        } else {
> -            sum += (uint32_t)buf[i - seq] << 8;
> -        }
> +    for (i = 0; i < len - 1; i += 2) {
> +        sum1 += (uint32_t)buf[i];
> +        sum2 += (uint32_t)buf[i + 1];
> +    }
> +    if (i < len) {
> +        sum1 += (uint32_t)buf[i];
> +    }
> +
> +    if (seq & 1) {
> +        return sum1 + (sum2 << 8);
> +    } else {
> +        return sum2 + (sum1 << 8);
>     }
> -    return sum;
> }
> 
> uint16_t net_checksum_finish(uint32_t sum)
> -- 
> 2.7.4
>