Re: [Qemu-devel] [PATCHv4 0/9] buffer_is_zero / migration optimizations

[Paolo Bonzini]

Il 22/03/2013 20:20, Peter Lieven ha scritto:
>> I think patch 4 is a bit overengineered.  I would prefer the simple
>> patch you had using three/four non-vectorized accesses.  The setup cost
>> of the vectorized buffer_is_zero is quite high, and 64 bits are just
>> 256k RAM; if the host doesn't touch 256k RAM, it will incur the overhead.
> I think you are right. I was a little to eager to utilize 
> buffer_find_nonzero_offset()
> as much as possible. The performance gain by unrolling was impressive enough.
> The gain by the vector functions is not that big that it would justify a 
> possible
> slow down by the high setup costs. My testings revealed that in most cases 
> buffer_find_nonzero_offset()
> returns 0 or a big offset. All the 0 return values would have increased setup 
> costs with
> the vectorized version of patch 4.
> 
>>
>> I would prefer some more benchmarking for patch 5, but it looks ok.
> What would you like to see? Statistics how many pages of a real system
> are not zero, but zero in the first sizeof(long) bytes?

Yeah, more or less.  Running the system for a while, migrating, and
plotting a histogram of the return values of buffer_find_nonzero_offset
(hmm, perhaps using a nonvectorized version is better for this experiment).

Paolo