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

[Anthony Liguori]

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<a...@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;
  }