[Qemu-devel] [RFC] find_next_bit optimizations

[Peter Lieven <pl@dlhnet.de>]

Hi,

I ever since had a few VMs which are very hard to migrate because of a lot of memory I/O. I found that finding the next dirty bit
seemed to be one of the culprits (apart from removing locking which Paolo is working on).

I have to following proposal which seems to help a lot in my case. Just wanted to have some feedback.
I applied the same unrolling idea like in buffer_is_zero().

Peter

--- a/util/bitops.c
+++ b/util/bitops.c
@@ -24,12 +24,13 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
      const unsigned long *p = addr + BITOP_WORD(offset);
      unsigned long result = offset & ~(BITS_PER_LONG-1);
      unsigned long tmp;
+    unsigned long d0,d1,d2,d3;

      if (offset >= size) {
          return size;
      }
      size -= result;
-    offset %= BITS_PER_LONG;
+    offset &= (BITS_PER_LONG-1);
      if (offset) {
          tmp = *(p++);
          tmp &= (~0UL << offset);
@@ -43,6 +44,18 @@ unsigned long find_next_bit(const unsigned long *addr, unsigned long size,
          result += BITS_PER_LONG;
      }
      while (size & ~(BITS_PER_LONG-1)) {
+        while (!(size & (4*BITS_PER_LONG-1))) {
+            d0 = *p;
+            d1 = *(p+1);
+            d2 = *(p+2);
+            d3 = *(p+3);
+            if (d0 || d1 || d2 || d3) {
+                break;
+            }
+            p+=4;
+            result += 4*BITS_PER_LONG;
+            size -= 4*BITS_PER_LONG;
+        }
          if ((tmp = *(p++))) {
              goto found_middle;
          }