Re: [Qemu-devel] [PATCH RFC kernel] balloon: speed up inflating/deflating process

[Michael S. Tsirkin]

On Wed, May 25, 2016 at 10:10:47AM +0000, Li, Liang Z wrote:
> > > > >
> > > > > Hi MST,
> > > > >
> > > > > I have measured the performance when using a 32K page bitmap,
> > > >
> > > > Just to make sure. Do you mean a 32Kbyte bitmap?
> > > > Covering 1Gbyte of memory?
> > > Yes.
> > >
> > > >
> > > > > and inflate the balloon to 3GB
> > > > > of an idle guest with 4GB RAM.
> > > >
> > > > Should take 3 requests then, right?
> > > >
> > >
> > > No,  we can't assign the PFN when allocating page in balloon driver,
> > > So the PFNs of pages allocated may be across a large range,  we will
> > > tell the host once the pfn_max -pfn_min >= 0x40000(1GB range), so the
> > > requests count is most likely to be more than 3.
> > >
> > > > > Now:
> > > > > total inflating time: 338ms
> > > > > the count of virtio data transmission:  373
> > > >
> > > > Why was this so high? I would expect 3 transmissions.
> > >
> > > I follow your suggestion:
> > > ----------------------------------------------------------------------
> > > -------------- Suggestion to address all above comments:
> > >   1. allocate a bunch of pages and link them up,
> > >      calculating the min and the max pfn.
> > >      if max-min exceeds the allocated bitmap size,
> > >      tell host.
> > >   2. limit allocated bitmap size to something reasonable.
> > >      How about 32Kbytes? This is 256kilo bit in the map, which comes
> > >      out to 1Giga bytes of memory in the balloon.
> > > ----------------------------------------------------------------------
> > > --------------- Because the PFNs of the allocated pages are not linear
> > > increased, so 3 transmissions are  impossible.
> > >
> > >
> > > Liang
> > 
> > Interesting. How about instead of tell host, we do multiple scans, each time
> > ignoring pages out of range?
> > 
> >     for (pfn = min pfn; pfn < max pfn; pfn += 1G) {
> >             foreach page
> >                     if page pfn < pfn || page pfn >= pfn + 1G
> >                             continue
> >                     set bit
> >             tell host
> >     }
> > 
> 
> That means we have to allocate/free all the requested pages first, and then 
> tell the host.
> It works fine for inflating, but for deflating, because the page has been 
> deleted from the vb-> vb_dev_info->pages,
> so, we have to use a struct to save the dequeued pages before calling 
> release_pages_balloon(),

struct list_head?  I think you can just replace set_page_pfns with
        list_add(&page->lru, &page_list);



>  I think a page bitmap is the best struct to save these pages, because it 
> consumes less memory.
> And that bitmap should be large enough to save pfn 0 to max_pfn.
> 
> If the above is true, then we are back to the square one. we really need a 
> large page bitmap. Right?
> 
> Liang

These look like implementation issues to me.

I think the below might be helpful (completely untested),
your work can go on top.

--->

virtio-balloon: rework deflate to add page to a tmp list

Will allow faster notifications using a bitmap down the road.

Signed-off-by: Michael S. Tsirkin <address@hidden>

---

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 476c0e3..44050a3 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -195,8 +195,9 @@ static void release_pages_balloon(struct virtio_balloon *vb)
 static unsigned leak_balloon(struct virtio_balloon *vb, size_t num)
 {
        unsigned num_freed_pages;
-       struct page *page;
+       struct page *page, *next;
        struct balloon_dev_info *vb_dev_info = &vb->vb_dev_info;
+       LIST_HEAD(pages);               /* Pages dequeued for handing to Host */
 
        /* We can only do one array worth at a time. */
        num = min(num, ARRAY_SIZE(vb->pfns));
@@ -207,10 +208,13 @@ static unsigned leak_balloon(struct virtio_balloon *vb, 
size_t num)
                page = balloon_page_dequeue(vb_dev_info);
                if (!page)
                        break;
-               set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+               list_add(&page->lru, &pages);
                vb->num_pages -= VIRTIO_BALLOON_PAGES_PER_PAGE;
        }
 
+       list_for_each_entry_safe(page, next, &pages, lru)
+               set_page_pfns(vb, vb->pfns + vb->num_pfns, page);
+
        num_freed_pages = vb->num_pfns;
        /*
         * Note that if
-- 
MST