[Qemu-devel] Re: [PATCH] qemu-kvm: Speed up of the dirty-bitmap-traveling

[Avi Kivity]

On 02/05/2010 12:18 PM, OHMURA Kei wrote:
> dirty-bitmap-traveling is carried out by byte size in qemu-kvm.c.
> But We think that dirty-bitmap-traveling by long size is faster than by byte
> size especially when most of memory is not dirty.
>
>
>
> +
> +static int kvm_get_dirty_pages_log_range_by_long(unsigned long start_addr,
> +                                                 unsigned char *bitmap,
> +                                                 unsigned long offset,
> +                                                 unsigned long mem_size)
> +{
> +    unsigned int i;
> +    unsigned int len;
> +    unsigned long *bitmap_ul = (unsigned long *)bitmap;
> +
> +    /* bitmap-traveling by long size is faster than by byte size
> +     * especially when most of memory is not dirty.
> +     * bitmap should be long-size aligned for traveling by long.
> +     */
> +    if (((unsigned long)bitmap & (TARGET_LONG_SIZE - 1)) == 0) {
>   

Since we allocate the bitmap, we can be sure that it is aligned on a
long boundary (qemu_malloc() should guarantee that). So you can
eliminate the fallback.

> +        len = ((mem_size / TARGET_PAGE_SIZE) + TARGET_LONG_BITS - 1) /
> +            TARGET_LONG_BITS;
> +        for (i = 0; i < len; i++)
> +            if (bitmap_ul[i] != 0)
> +                kvm_get_dirty_pages_log_range_by_byte(i * TARGET_LONG_SIZE, 
> +                    (i + 1) * TARGET_LONG_SIZE, bitmap, offset);
>   

Better to just use the original loop here (since we don't need the
function as a fallback).

> +        /* 
> +         * We will check the remaining dirty-bitmap, 
> +         * when the mem_size is not a multiple of TARGET_LONG_SIZE. 
> +         */ 
> +        if ((mem_size & (TARGET_LONG_SIZE - 1)) != 0) {
> +            len = ((mem_size / TARGET_PAGE_SIZE) + 7) / 8;
> +            kvm_get_dirty_pages_log_range_by_byte(i * TARGET_LONG_SIZE, 
> +                len, bitmap, offset);
> +        }
>   

Seems like the bitmap size is also aligned as well (allocated using
BITMAP_SIZE which aligns using HOST_LONG_BITS), so this is unnecessary
as well.

-- 
error compiling committee.c: too many arguments to function