Il 11/03/2013 14:44, Peter Lieven ha scritto:
> 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))) {
This really means
if (!(size & (4*BITS_PER_LONG-1))) {
while (1) {
...
}
}
because the subtraction will not change the result of the "while" loop
condition.
What you want is probably "while (size & ~(4*BITS_PER_LONG-1))", which
in turn means "while (size >= 4*BITS_PER_LONG).
Please change both while loops to use a ">=" condition, it's easier to read.
Paolo
> + 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;
> }
