RE: [PATCH v2 repost 4/7] virtio-balloon: speed up inflate/deflate process

Li, Liang Z
2016-07-28T03:06:37Z

> > + * VIRTIO_BALLOON_PFNS_LIMIT is used to limit the size of page bitmap
> > + * to prevent a very large page bitmap, there are two reasons for this:
> > + * 1) to save memory.
> > + * 2) allocate a large bitmap may fail.
> > + *
> > + * The actual limit of pfn is determined by:
> > + * pfn_limit = min(max_pfn, VIRTIO_BALLOON_PFNS_LIMIT);
> > + *
> > + * If system has more pages than VIRTIO_BALLOON_PFNS_LIMIT, we will
> > +scan
> > + * the page list and send the PFNs with several times. To reduce the
> > + * overhead of scanning the page list. VIRTIO_BALLOON_PFNS_LIMIT
> > +should
> > + * be set with a value which can cover most cases.
>
> So what if it covers 1/32 of the memory? We'll do 32 exits and not 1, still not a
> big deal for a big guest.
>

The issue here is the overhead is too high for scanning the page list for 32 times.
Limit the page bitmap size to a fixed value is better for a big guest?

> > + */
> > +#define VIRTIO_BALLOON_PFNS_LIMIT ((32 * (1ULL << 30)) >>
> PAGE_SHIFT)
> > +/* 32GB */
>
> I already said this with a smaller limit.
>
> 2<< 30 is 2G but that is not a useful comment.
> pls explain what is the reason for this selection.
>
> Still applies here.
>

I will add the comment for this.

> > - sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
> > + if (virtio_has_feature(vb->vdev,
> VIRTIO_BALLOON_F_PAGE_BITMAP)) {
> > + struct balloon_bmap_hdr *hdr = vb->bmap_hdr;
> > + unsigned long bmap_len;
> > +
> > + /* cmd and req_id are not used here, set them to 0 */
> > + hdr->cmd = cpu_to_virtio16(vb->vdev, 0);
> > + hdr->page_shift = cpu_to_virtio16(vb->vdev, PAGE_SHIFT);
> > + hdr->reserved = cpu_to_virtio16(vb->vdev, 0);
> > + hdr->req_id = cpu_to_virtio64(vb->vdev, 0);
>
> no need to swap 0, just fill it in. in fact you allocated all 0s so no need to touch
> these fields at all.
>

Will change in v3.

> > @@ -489,7 +612,7 @@ static int virtballoon_migratepage(struct
> > balloon_dev_info *vb_dev_info, static int virtballoon_probe(struct
> > virtio_device *vdev) {
> > struct virtio_balloon *vb;
> > - int err;
> > + int err, hdr_len;
> >
> > if (!vdev->config->get) {
> > dev_err(&vdev->dev, "%s failure: config access disabled\n",
> @@
> > -508,6 +631,18 @@ static int virtballoon_probe(struct virtio_device *vdev)
> > spin_lock_init(&vb->stop_update_lock);
> > vb->stop_update = false;
> > vb->num_pages = 0;
> > + vb->pfn_limit = VIRTIO_BALLOON_PFNS_LIMIT;
> > + vb->pfn_limit = min(vb->pfn_limit, get_max_pfn());
> > + vb->bmap_len = ALIGN(vb->pfn_limit, BITS_PER_LONG) /
> > + BITS_PER_BYTE + 2 * sizeof(unsigned long);
>
> What are these 2 longs in aid of?
>
The rounddown(vb->start_pfn, BITS_PER_LONG) and roundup(vb->end_pfn, BITS_PER_LONG)
may cause (vb->end_pfn - vb->start_pfn) > vb->pfn_limit, so we need extra space to save the
bitmap for this case. 2 longs are enough.

> > + hdr_len = sizeof(struct balloon_bmap_hdr);
> > + vb->bmap_hdr = kzalloc(hdr_len + vb->bmap_len, GFP_KERNEL);
>
> So it can go up to 1MByte but adding header size etc you need a higher order
> allocation. This is a waste, there is no need to have a power of two allocation.
> Start from the other side. Say "I want to allocate 32KBytes for the bitmap".
> Subtract the header and you get bitmap size.
> Calculate the pfn limit from there.
>

Indeed, will change. Thanks a lot!

Liang