linux-kernel - Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of VIRTIO_BALLOON_F_CHUNK

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Date:   Thu, 9 Mar 2017 06:14:11 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Wei Wang <wei.w.wang@...el.com>
Cc:     virtio-dev@...ts.oasis-open.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, linux-mm@...ck.org,
        Liang Li <liang.z.li@...el.com>,
        "Michael S . Tsirkin" <mst@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Cornelia Huck <cornelia.huck@...ibm.com>,
        Amit Shah <amit.shah@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Liang Li <liliang324@...il.com>
Subject: Re: [PATCH v7 kernel 3/5] virtio-balloon: implementation of
 VIRTIO_BALLOON_F_CHUNK_TRANSFER

On Fri, Mar 03, 2017 at 01:40:28PM +0800, Wei Wang wrote:
> From: Liang Li <liang.z.li@...el.com>
> 1) allocating pages (6.5%)
> 2) sending PFNs to host (68.3%)
> 3) address translation (6.1%)
> 4) madvise (19%)
> 
> This patch optimizes step 2) by transfering pages to the host in
> chunks. A chunk consists of guest physically continuous pages, and
> it is offered to the host via a base PFN (i.e. the start PFN of
> those physically continuous pages) and the size (i.e. the total
> number of the pages). A normal chunk is formated as below:
> -----------------------------------------------
> |  Base (52 bit)               | Size (12 bit)|
> -----------------------------------------------
> For large size chunks, an extended chunk format is used:
> -----------------------------------------------
> |                 Base (64 bit)               |
> -----------------------------------------------
> -----------------------------------------------
> |                 Size (64 bit)               |
> -----------------------------------------------

What's the advantage to extended chunks?  IOW, why is the added complexity
of having two chunk formats worth it?  You already reduced the overhead by
a factor of 4096 with normal chunks ... how often are extended chunks used
and how much more efficient are they than having several normal chunks?