[Qemu-devel] [PATCH v2] Optimize screendump

[Avi Kivity <avi@redhat.com>]

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>
---

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;
 }
-- 
1.7.5.3