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

[Paolo Bonzini <pbonzini@redhat.com>]

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;

> 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.

Paolo