Re: [PATCH 0/1] KVM: make get dirty log ioctl return the first dirty page's position

[Avi Kivity <avi@redhat.com>]

On 02/24/2010 10:55 AM, Avi Kivity wrote:
> On 02/24/2010 10:43 AM, Takuya Yoshikawa wrote:
>> Some weeks ago, OHMURA Kei revised the qemu-kvm's
>> dirty bitmap scan by accessing the bitmap as an
>> unsigned long array.
>>
>> By reviewing this work more, we notice that kernel
>> side is doing a similar thing to check the bitmap is
>> all clean or not.
>>
>> So I made a patch which makes the get dirty log ioctl
>> return the first dirty page position found by this check.
>>
>> Though my test is not enough to show the effect of this
>> patch, the fact that this patch has no bad effect to both
>> performance and implementation logic and we can skip some
>> extra memory accesses and comparisons in userspace seems
>> to be suggesting this patch is promising, right?
>
> Well, if 10% of the pages are dirty, the new ioctl will statistically 
> return something within the first 20% of the slot, so we can skip 10% 
> and have to do the next 90%.  Given that we already walked the bitmap 
> once in the kernel and the saving is only in userspace, the average 
> saving in bitmap-walking is only 5%.
>
> The patch's greatest benefit is if all pages are clean (100% saved) or 
> if just one page is dirty (50% saved) but that will be very rare.  So 
> I think the return-on-churn here is too low.

btw, one idea I had was to allocate the bitmap in userspace and let the 
kernel set bits directly.  This reduces the amount of unswappable memory 
the kernel allocates and reduces copying.

A problem with this is that userspace cannot just clear the bits, since 
the kernel has to write-protect the pages again.  I don't know how we 
can do this without copying the bitmap.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.