Re: [Qemu-devel] [PATCH] migration: not send zero page header in ram bulk stage

[Li, Liang Z]

> * Liang Li (address@hidden) wrote:
> > Now that VM's RAM pages are initialized to zero, (VM's RAM is allcated
> > with the mmap() and MAP_ANONYMOUS option, or mmap() without
> MAP_SHARED
> > if hugetlbfs is used.) so there is no need to send the zero page
> > header to destination.
> >
> > For guest just uses a small portions of RAM, this change can avoid
> > allocating all the guest's RAM pages in the destination node after
> > live migration. Another benefit is destination QEMU can save lots of
> > CPU cycles for zero page checking.
> 
> I think this would break postcopy, because the zero pages wouldn't be filled
> in, so accessing them would still generate a userfault.
> So you'd have to disable this optimisation if postcopy is enabled (even during
> the precopy bulk stage).
> 
> Also, are you sure about the benefits?
>  Destination guests RAM should not be allocated on receiving a zero page;
> see ram_handle_compressed, it doesn't write to the page if it's zero, so it
> shouldn't cause an allocate.  I think you're probably correct about the zero
> page test on the destination, I wonder if we can speed that up.
> 
> Dave

I have test the performance, with a 8G guest just booted, this patch can reduce 
total live migration time about 10%.
Unfortunately, Paolo said this patch would break LM in some case ....

For the zero page test on the destination, if the page is really a zero page, 
test is faster than writing a whole page of zero.

Liang