Re: [RFC PATCH 1/2] KVM: count the number of dirty bits for each memslot

[Avi Kivity <avi@redhat.com>]

On 11/18/2010 03:10 PM, Jan Kiszka wrote:
> Am 18.11.2010 13:54, Avi Kivity wrote:
> >  On 11/18/2010 07:14 AM, Takuya Yoshikawa wrote:
> >>  This patch introduces the counter to hold the number of dirty bits in
> >>  each
> >>  memslot. We will use this to optimize dirty logging later.
> >>
> >>
> >>  @@ -3217,11 +3216,8 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm,
> >>
> >>        n = kvm_dirty_bitmap_bytes(memslot);
> >>
> >>  -    for (i = 0; !is_dirty&&   i<   n/sizeof(long); i++)
> >>  -        is_dirty = memslot->dirty_bitmap[i];
> >>  -
> >
> >  This can already be an improvement.
>
> /Me wonders if it wouldn't make sense to expand this optimization to the
> user space interface as well, i.e. signaling "there are no dirty pages"
> via some flag instead of writing zeros in a bitmap. Of course, this
> means supporting both interfaces for a longer period.

An 8MB framebuffer is 2K bits, or 256 bytes wide.  Comparing 256 cache 
hot bytes against zero is not worth a new interface.

Larger memory slots are very unlikely to be always unmodified.

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