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

[Michael S. Tsirkin]

On Tue, May 24, 2016 at 02:36:08PM +0000, Li, Liang Z wrote:
> > > > > > This can be pre-initialized, correct?
> > > > >
> > > > > pre-initialized? I am not quite understand your mean.
> > > >
> > > > I think you can maintain sg as part of device state and init sg with the
> > bitmap.
> > > >
> > >
> > > I got it.
> > >
> > > > > > This is grossly inefficient if you only requested a single page.
> > > > > > And it's also allocating memory very aggressively without ever
> > > > > > telling the host what is going on.
> > > > >
> > > > > If only requested a single page, there is no need  to send the
> > > > > entire page bitmap, This RFC patch has already considered about this.
> > > >
> > > > where's that addressed in code?
> > > >
> > >
> > > By record the start_pfn and end_pfn.
> > >
> > > The start_pfn & end_pfn will be updated in set_page_bitmap() and will
> > > be used in the function tell_host():
> > >
> > > ----------------------------------------------------------------------
> > > -----------
> > > +static void set_page_bitmap(struct virtio_balloon *vb, struct page
> > > +*page) {
> > > + unsigned int i;
> > > + unsigned long *bitmap = vb->page_bitmap;
> > > + unsigned long balloon_pfn = page_to_balloon_pfn(page);
> > > +
> > > + for (i = 0; i < VIRTIO_BALLOON_PAGES_PER_PAGE; i++)
> > > +         set_bit(balloon_pfn + i, bitmap);
> > 
> > BTW, there's a page size value in header so there is no longer need to set
> > multiple bits per page.
> 
> Yes, you are right.
> 
> > 
> > > + if (balloon_pfn < vb->start_pfn)
> > > +         vb->start_pfn = balloon_pfn;
> > > + if (balloon_pfn > vb->end_pfn)
> > > +         vb->end_pfn = balloon_pfn;
> > > +}
> > 
> > Sounds good, but you also need to limit by allocated bitmap size.
> 
> Why should we limit the page bitmap size? Is it no good to send a large page 
> bitmap?
> or to save the memory used for page bitmap? Or some other reason?

To save memory. First allocating a large bitmap can fail, second this is
pinned memory that is wasted - it's unused most of the time while guest
is running.

> > 
> > >
> > > +         unsigned long start_pfn, end_pfn, flags = 0, bmap_len;
> > > +         struct scatterlist sg[5];
> > > +
> > > +         start_pfn = rounddown(vb->start_pfn, BITS_PER_LONG);
> > > +         end_pfn = roundup(vb->end_pfn, BITS_PER_LONG);
> > > +         bmap_len = (end_pfn - start_pfn) / BITS_PER_LONG *
> > sizeof(long);
> > > +
> > > +         sg_init_table(sg, 5);
> > > +         sg_set_buf(&sg[0], &flags, sizeof(flags));
> > > +         sg_set_buf(&sg[1], &start_pfn, sizeof(start_pfn));
> > > +         sg_set_buf(&sg[2], &page_shift, sizeof(page_shift));
> > > +         sg_set_buf(&sg[3], &bmap_len, sizeof(bmap_len));
> > > +         sg_set_buf(&sg[4], vb->page_bitmap +
> > > +                          (start_pfn / BITS_PER_LONG), bmap_len);
> > 
> > Looks wrong. start_pfn should start at offset 0 I think ...
> 
> I don't know what is wrong here, could you tell me why?
> 
> Thanks!
> 
> Liang

start_pfn should mean "bit 0 in bitmap refers to pfn X".
So it does not make sense to also add it as offset within bitmap.

-- 
MST