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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 25/03/2013 14:32, Peter Lieven ha scritto:
> 
> Am 25.03.2013 um 14:23 schrieb Peter Lieven <pl@kamp.de>:
> 
>>
>> Am 25.03.2013 um 14:02 schrieb Paolo Bonzini <pbonzini@redhat.com>:
>>
>>>> Maybe I should have explained the output more detailed. The percentages
>>>> are added. 35.8% in the second last column means that
>>>> 35.8% have a return value that is less than TARGET_PAGE_SIZE.
>>>> This was meant to illustrate at how many 64-bit chunks you have
>>>> to look to grab a certain percentage of non-zero pages.
>>>
>>> Ok, I wrongly understood that many pages had 4088 zero bytes but
>>> the last 8 were not zero.  Now it's clearer, and more logical too. :)
>>>
>>>> Looking e.g. at the third value it means that looking at the first
>>>> three 64-bit chunks it will catch 34.0% of all pages.
>>>> It turns out that the non-zeroness of a page can be detected looking
>>>> at the first 256 or so bits and only a low
>>>> percentage turns out to be non-zero at a later position. So after
>>>> having checked the first chunks one by one
>>>> there is no big penalty looking at the remaining chunks with the
>>>> vectorized loop.
>>>
>>> I think it makes most sense to unroll the first four non-vectorized
>>> iterations, i.e. not use SSE and use three or four ifs.  Either:
>>>
>>>  if (foo[0]) return 0;
>>>  if (foo[1]) return 8;
>>>  if (foo[2]) return 16;
>>>  if (foo[3]) return 24;
>>>
>>> or
>>>
>>>  if (foo[0]) return 0;
>>>  if (foo[1] | foo[2] | foo[3]) return 8;
>>>
>>> and then proceed on the remaining 4096-4*sizeof(long) bytes with
>>> the vectorized loop.  foo+4 is aligned for SIMD operations on both
>>> 32- and 64-bit machines, which makes this a nice choice.
>>
>> i can't start at foo+4 since the remaining X-4*sizeof(long) bytes
>> are not dividable by 8*sizeof(VECTYPE).


Hmm, right.  What about just processing the first few longs twice, i.e.
the above followed by "for (i = 0; i < len / sizeof(sizeof(VECTYPE); i
+= BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR)"?

Paolo

>>
>>    for (i = BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR; 
>>         i < len / sizeof(VECTYPE); 
>>         i += BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR) {
>>        …
>>    }
> 
> performance of the above is bad compared to:
> 
>     for (i = 0; i < BUFFER_FIND_NONZERO_OFFSET_UNROLL_FACTOR; i++) {
>         if (!ALL_EQ(p[i], zero)) {
>             return i * sizeof(VECTYPE);
>         }
>     }
> 
> …
> 
> The above is basically what old is_dup_page is doing, but after the first
> 8 iterations the optimized version kicks in.
> 
> Peter
> 
> 
>