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    SubjectRE: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization
    Date
    > > 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.

    > > 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

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