Re: [Qemu-devel] [PATCH] migration: vectorize is_dup_page

[Anthony Liguori]

On 12/06/2011 11:25 AM, Paolo Bonzini wrote:
> is_dup_page is already proceeding in 32-bit chunks.  Changing it to 16
> bytes using Altivec or SSE is easy, and provides a noticeable improvement.
> Pierre Riteau measured 30->25 seconds on a 16GB guest, I measured 4.6->3.9
> seconds on a 6GB guest (best of three times for me; dunno for Pierre).
> Both of them are approximately a 15% improvement.
>
> I tried playing with non-temporal prefetches, but I did not get any
> improvement (though I did get less cache misses, so the patch was doing
> its job).
>
> Signed-off-by: Paolo Bonzini<address@hidden>
> ---
>   arch_init.c |   28 ++++++++++++++++++++++------
>   1 files changed, 22 insertions(+), 6 deletions(-)
>
> diff --git a/arch_init.c b/arch_init.c
> index cdad805..473df2d 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -94,14 +94,30 @@ const uint32_t arch_type = QEMU_ARCH;
>   #define RAM_SAVE_FLAG_EOS      0x10
>   #define RAM_SAVE_FLAG_CONTINUE 0x20
>
> -static int is_dup_page(uint8_t *page, uint8_t ch)
> +#if __ALTIVEC__

I think you want #ifdefs here and possibly below:

  CC    x86_64-softmmu/arch_init.o
cc1: warnings being treated as errors
/home/anthony/git/qemu/arch_init.c:97:5: error: "__ALTIVEC__" is not defined
/home/anthony/git/qemu/arch_init.c: In function ‘is_dup_page’:
/home/anthony/git/qemu/arch_init.c:116:5: error: incompatible type for argument 
1 of ‘_mm_set1_epi8’
/usr/lib/x86_64-linux-gnu/gcc/x86_64-linux-gnu/4.5.2/include/emmintrin.h:636:1: 
note: expected ‘char’ but argument is of type ‘__m128i’

Regards,

Anthony Liguori

> +#include<altivec.h>
> +#define VECTYPE        vector unsigned char
> +#define SPLAT(p)       vec_splat(vec_ld(0, p), 0)
> +#define ALL_EQ(v1, v2) vec_all_eq(v1, v2)
> +#elif __SSE2__
> +#include<emmintrin.h>
> +#define VECTYPE        __m128i
> +#define SPLAT(p)       _mm_set1_epi8(*(p))
> +#define ALL_EQ(v1, v2) (_mm_movemask_epi8(_mm_cmpeq_epi8(v1, v2)) == 0xFFFF)
> +#else
> +#define VECTYPE        unsigned long
> +#define SPLAT(p)       (*(p) * (~0UL / 255))
> +#define ALL_EQ(v1, v2) ((v1) == (v2))
> +#endif
> +
> +static int is_dup_page(uint8_t *page)
>   {
> -    uint32_t val = ch<<  24 | ch<<  16 | ch<<  8 | ch;
> -    uint32_t *array = (uint32_t *)page;
> +    VECTYPE *p = (VECTYPE *)page;
> +    VECTYPE val = SPLAT(p);
>       int i;
>
> -    for (i = 0; i<  (TARGET_PAGE_SIZE / 4); i++) {
> -        if (array[i] != val) {
> +    for (i = 0; i<  TARGET_PAGE_SIZE / sizeof(VECTYPE); i++) {
> +        if (!ALL_EQ(val, p[i])) {
>               return 0;
>           }
>       }
> @@ -136,7 +152,7 @@ static int ram_save_block(QEMUFile *f)
>
>               p = block->host + offset;
>
> -            if (is_dup_page(p, *p)) {
> +            if (is_dup_page(p)) {
>                   qemu_put_be64(f, offset | cont | RAM_SAVE_FLAG_COMPRESS);
>                   if (!cont) {
>                       qemu_put_byte(f, strlen(block->idstr));