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

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Date:	Fri, 20 May 2016 12:32:43 +0200
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	Liang Li <liang.z.li@...el.com>
Cc:	mst@...hat.com, kvm@...r.kernel.org, qemu-devel@...gnu.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, amit.shah@...hat.com,
	pbonzini@...hat.com, akpm@...ux-foundation.org, dgilbert@...hat.com
Subject: Re: [PATCH RFC kernel] balloon: speed up inflating/deflating
 process

On Fri, 20 May 2016 17:59:46 +0800
Liang Li <liang.z.li@...el.com> 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@...el.com>
> ---
>  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.)