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

[Peter Lieven <pl@kamp.de>]

Am 22.03.2013 um 22:24 schrieb Paolo Bonzini <pbonzini@redhat.com>:

> 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).

I will follow up with this on Monday. Have you seen my concern, that the whole
page is read anyway if it is non-zero?

Peter


> 
> Paolo