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

[Peter Lieven]

Am 25.03.2013 um 11:53 schrieb Paolo Bonzini <address@hidden>:

>> ubuntu 12.04 LTS 64-bit desktop with 1G memory shortly after boot:
>> histogram: 31.7% 32.9% [...] 36.4% 100.0%
>> 
>> ---
>> 
>> opensuse 11.1 64-bit with 24GB ram (busy server)
>> histogram: 97.5% 97.9% [...] 99.5% 100.0%
>> 
>> ---
>> 
>> windows server 2008 R2 with 8G ram running for 3 days:
>> histogram:  20.9% 21.3% [...] 22.5% 100.0%
>> 
>> ---
>> 
>> windows XP guest with 1G Ram running for approx. 1 hours
>> histogram: 25.6% [...] 35.8% 100.0%
> 
> Doesn't this suggest checking the first _and the last_ word,
> and using the vectorized loop if none is zero?

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.

25.6% 26.1% 34.0% 34.1% 34.2% 34.3% 34.3% 34.4% 34.4% 34.4% 34.5% 34.5% 34.5% 
[...] 35.8% 100%

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.

Here is the distribution of return values for the Windows XP example:

25.62% 0.49% 7.86% 0.12% 0.15% 0.05% 0.05% 0.04% 0.05% 0.02% 0.03% 0.02% 0.03% 
0.02% 0.02% 0.01% 0.03% 0.02% 0.01% 0.02% 0.02% 0.01% 0.02% 0.01% 0.01% 0.01% 
0.01% 0.01% 0.02% 0.00% 0.01% 0.02% 0.03% 0.01% 0.01% 0.01% 0.01% 0.01% 0.01% 
0.00% 0.00% 0.01% 0.07% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.01% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.01% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.01% 0.02% 0.00% 0.00% 0.00% 0.00% 
0.01% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.02% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 
0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.03% 0.00% 
0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.02% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.01% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.02% 0.00% 0.00% 0.02% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.01% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.01% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.01% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 0.00% 
0.00% 0.00% 0.00% 0.00% 0.00% 64.23%

The last value is the percentage of return value of TARGET_PAGE_SIZE meaning 
the page is all zero.

Peter