Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization

[Michael S. Tsirkin]

On Fri, Mar 04, 2016 at 03:13:03PM +0000, Li, Liang Z wrote:
> > > Maybe I am not clear enough.
> > >
> > > I mean if we inflate balloon before live migration, for a 8GB guest, it takes
> > about 5 Seconds for the inflating operation to finish.
> > 
> > And these 5 seconds are spent where?
> > 
> 
> The time is spent on allocating the pages and send the allocated pages pfns to QEMU
> through virtio.

What if we skip allocating pages but use the existing interface to send pfns
to QEMU?

> > > For the PV solution, there is no need to inflate balloon before live
> > > migration, the only cost is to traversing the free_list to  construct
> > > the free pages bitmap, and it takes about 20ms for a 8GB idle guest( less if
> > there is less free pages),  passing the free pages info to host will take about
> > extra 3ms.
> > >
> > >
> > > Liang
> > 
> > So now let's please stop talking about solutions at a high level and discuss the
> > interface changes you make in detail.
> > What makes it faster? Better host/guest interface? No need to go through
> > buddy allocator within guest? Less interrupts? Something else?
> > 
> 
> I assume you are familiar with the current virtio-balloon and how it works. 
> The new interface is very simple, send a request to the virtio-balloon driver,
> The virtio-driver will travers the '&zone->free_area[order].free_list[t])' to 
> construct a 'free_page_bitmap', and then the driver will send the content
> of  'free_page_bitmap' back to QEMU. That all the new interface does and
> there are no ' alloc_page' related affairs, so it's faster.
> 
> 
> Some code snippet:
> ----------------------------------------------
> +static void mark_free_pages_bitmap(struct zone *zone,
> +		 unsigned long *free_page_bitmap, unsigned long pfn_gap) {
> +	unsigned long pfn, flags, i;
> +	unsigned int order, t;
> +	struct list_head *curr;
> +
> +	if (zone_is_empty(zone))
> +		return;
> +
> +	spin_lock_irqsave(&zone->lock, flags);
> +
> +	for_each_migratetype_order(order, t) {
> +		list_for_each(curr, &zone->free_area[order].free_list[t]) {
> +
> +			pfn = page_to_pfn(list_entry(curr, struct page, lru));
> +			for (i = 0; i < (1UL << order); i++) {
> +				if ((pfn + i) >= PFN_4G)
> +					set_bit_le(pfn + i - pfn_gap,
> +						   free_page_bitmap);
> +				else
> +					set_bit_le(pfn + i, free_page_bitmap);
> +			}
> +		}
> +	}
> +
> +	spin_unlock_irqrestore(&zone->lock, flags); }
> ----------------------------------------------------
> Sorry for my poor English and expression, if you still can't understand,
> you could glance at the patch, total about 400 lines.
> > 
> > > > --
> > > > MST