Re: [Qemu-devel] [PATCH v2] Optimize screendump

[Anthony Liguori <anthony@codemonkey.ws>]

On 06/20/2011 03:12 AM, Avi Kivity wrote:
> When running kvm-autotest, fputc() is often the second highest (sometimes #1)
> function showing up in a profile.  This is due to fputc() locking the file
> for every byte written.
>
> Optimize by buffering a line's worth of pixels and writing that out in a
> single call.
>
> Signed-off-by: Avi Kivity<avi@redhat.com>

Applied.  Thanks.

Regards,

Anthony Liguori

> ---
>
> v2: drop unportable fputc_unlocked
>
>   hw/vga.c |   13 ++++++++++---
>   1 files changed, 10 insertions(+), 3 deletions(-)
>
> diff --git a/hw/vga.c b/hw/vga.c
> index d5bc582..97c96bf 100644
> --- a/hw/vga.c
> +++ b/hw/vga.c
> @@ -2349,15 +2349,19 @@ int ppm_save(const char *filename, struct DisplaySurface *ds)
>       uint32_t v;
>       int y, x;
>       uint8_t r, g, b;
> +    int ret;
> +    char *linebuf, *pbuf;
>
>       f = fopen(filename, "wb");
>       if (!f)
>           return -1;
>       fprintf(f, "P6\n%d %d\n%d\n",
>               ds->width, ds->height, 255);
> +    linebuf = qemu_malloc(ds->width * 3);
>       d1 = ds->data;
>       for(y = 0; y<  ds->height; y++) {
>           d = d1;
> +        pbuf = linebuf;
>           for(x = 0; x<  ds->width; x++) {
>               if (ds->pf.bits_per_pixel == 32)
>                   v = *(uint32_t *)d;
> @@ -2369,13 +2373,16 @@ int ppm_save(const char *filename, struct DisplaySurface *ds)
>                   (ds->pf.gmax + 1);
>               b = ((v>>  ds->pf.bshift)&  ds->pf.bmax) * 256 /
>                   (ds->pf.bmax + 1);
> -            fputc(r, f);
> -            fputc(g, f);
> -            fputc(b, f);
> +            *pbuf++ = r;
> +            *pbuf++ = g;
> +            *pbuf++ = b;
>               d += ds->pf.bytes_per_pixel;
>           }
>           d1 += ds->linesize;
> +        ret = fwrite(linebuf, 1, pbuf - linebuf, f);
> +        (void)ret;
>       }
> +    qemu_free(linebuf);
>       fclose(f);
>       return 0;
>   }