Re: [Qemu-devel] [v2 0/2] add avx2 instruction optimization

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/11/2015 09:53, Li, Liang Z wrote:
>> On 12/11/2015 03:49, Li, Liang Z wrote:
>>> I am very surprised about the live migration performance  result when
>>> I use your ' memeqzero4_paolo' instead of these SSE2 Intrinsics to
>>> check the zero pages.
>>
>> What code were you using?  Remember I suggested using only unsigned long
>> checks, like
>>
>> 	unsigned long *p = ...
>> 	if (p[0] || p[1] || p[2] || p[3]
>> 	    || memcmp(p+4, p, size - 4 * sizeof(unsigned long)) != 0)
>> 		return BUFFER_NOT_ZERO;
>> 	else
>> 		return BUFFER_ZERO;
>>
> 
> I use the following code:
> 
> 
> bool memeqzero4_paolo(const void *data, size_t length)
> {
>      ...
> }

The code you used is very generic and not optimized for the kind of data
you see during migration, hence the existing code in QEMU fares better.

>>> The total live migration time increased about
>>> 8%!   Not decreased.  Although in the unit test your '
>>> memeqzero4_paolo'  has better performance, any idea?
>>
>> You only tested the case of zero pages.  But real pages usually are not zero,
>> even if they have a few zero bytes at the beginning.  It's very important to
>> optimize the initial check before the memcmp call.
>>
> 
> In the unit test, I only test zero pages too, and the performance of  'memeqzero4_paolo' is better.
> But when merged into QEMU, it caused performance drop. Why?

Because QEMU is not migrating zero pages only.

Paolo