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

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Date:	Fri, 20 May 2016 13:19:51 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Liang Li <liang.z.li@...el.com>, mst@...hat.com
Cc:	linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
	virtualization@...ts.linux-foundation.org,
	akpm@...ux-foundation.org, dgilbert@...hat.com,
	amit.shah@...hat.com, kvm@...r.kernel.org
Subject: Re: [PATCH RFC kernel] balloon: speed up inflating/deflating process

On 20/05/2016 11:59, Liang Li wrote:
> +
> +		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));

These four should probably be placed in a single struct and therefore a
single sg entry.  It might even be faster to place it together with the
bitmap, thus avoiding the use of indirect descriptors.

You should also test ballooning of a 64GB guest after filling in the
page cache, not just ballooning of a freshly booted 4GB guest.  This
will give you a much more sparse bitmap.  Still, the improvement in
sending PFNs to the host are impressive.

Thanks,

Paolo

> +		sg_set_buf(&sg[4], vb->page_bitmap +
> +				 (start_pfn / BITS_PER_LONG), bmap_len);
> +		virtqueue_add_outbuf(vq, sg, 5, vb, GFP_KERNEL);