On Fri, 20 May 2016 17:59:46 +0800
Liang Li <liang.z.li@xxxxxxxxx> wrote:
> The implementation of the current virtio-balloon is not very efficient,
> Bellow is test result of time spends on inflating the balloon to 3GB of
> a 4GB idle guest:
>
> a. allocating pages (6.5%, 103ms)
> b. sending PFNs to host (68.3%, 787ms)
> c. address translation (6.1%, 96ms)
> d. madvise (19%, 300ms)
>
> It takes about 1577ms for the whole inflating process to complete. The
> test shows that the bottle neck is the stage b and stage d.
>
> If using a bitmap to send the page info instead of the PFNs, we can
> reduce the overhead spends on stage b quite a lot. Furthermore, it's
> possible to do the address translation and do the madvise with a bulk
> of pages, instead of the current page per page way, so the overhead of
> stage c and stage d can also be reduced a lot.
>
> This patch is the kernel side implementation which is intended to speed
> up the inflating & deflating process by adding a new feature to the
> virtio-balloon device. And now, inflating the balloon to 3GB of a 4GB
> idle guest only takes 175ms, it's about 9 times as fast as before.
>
> TODO: optimize stage a by allocating/freeing a chunk of pages instead
> of a single page at a time.
Not commenting on the approach, but...
>
> Signed-off-by: Liang Li <liang.z.li@xxxxxxxxx>
> ---
> drivers/virtio/virtio_balloon.c | 199 ++++++++++++++++++++++++++++++++++--
> include/uapi/linux/virtio_balloon.h | 1 +
> mm/page_alloc.c | 6 ++
> 3 files changed, 198 insertions(+), 8 deletions(-)
>
> static void tell_host(struct virtio_balloon *vb, struct virtqueue *vq)
> {
> - struct scatterlist sg;
> unsigned int len;
>
> - sg_init_one(&sg, vb->pfns, sizeof(vb->pfns[0]) * vb->num_pfns);
> + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_PAGE_BITMAP)) {
> + u32 page_shift = PAGE_SHIFT;
> + 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);
> + virtqueue_add_outbuf(vq, sg, 5, vb, GFP_KERNEL);
> +
...you need to take care of the endianness of the data you put on the
queue, otherwise virtio-1 on big endian won't work. (There's just been
a patch for that problem.)
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