linux-kernel - Re: [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

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Date:   Wed, 7 Dec 2016 07:45:21 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     David Hildenbrand <david@...hat.com>,
        "Li, Liang Z" <liang.z.li@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Cc:     "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        "mhocko@...e.com" <mhocko@...e.com>,
        "mst@...hat.com" <mst@...hat.com>,
        "qemu-devel@...gnu.org" <qemu-devel@...gnu.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>,
        "dgilbert@...hat.com" <dgilbert@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH kernel v5 0/5] Extend virtio-balloon for fast
 (de)inflating & fast live migration

On 12/07/2016 07:42 AM, David Hildenbrand wrote:
> Am 07.12.2016 um 14:35 schrieb Li, Liang Z:
>>> Am 30.11.2016 um 09:43 schrieb Liang Li:
>>>> This patch set contains two parts of changes to the virtio-balloon.
>>>>
>>>> One is the change for speeding up the inflating & deflating process,
>>>> the main idea of this optimization is to use bitmap to send the page
>>>> information to host instead of the PFNs, to reduce the overhead of
>>>> virtio data transmission, address translation and madvise(). This can
>>>> help to improve the performance by about 85%.
>>>
>>> Do you have some statistics/some rough feeling how many consecutive
>>> bits are
>>> usually set in the bitmaps? Is it really just purely random or is
>>> there some
>>> granularity that is usually consecutive?
>>>
>>
>> I did something similar. Filled the balloon with 15GB for a 16GB idle
>> guest, by
>> using bitmap, the madvise count was reduced to 605. when using the
>> PFNs, the madvise count
>> was 3932160. It means there are quite a lot consecutive bits in the
>> bitmap.
>> I didn't test for a guest with heavy memory workload.
> 
> Would it then even make sense to go one step further and report {pfn,
> length} combinations?
> 
> So simply send over an array of {pfn, length}?

Li's current patches do that.  Well, maybe not pfn/length, but they do
take a pfn and page-order, which fits perfectly with the kernel's
concept of high-order pages.

> And it makes sense if you think about:
> 
> a) hugetlb backing: The host may only be able to free huge pages (we
> might want to communicate that to the guest later, that's another
> story). Still we would have to send bitmaps full of 4k frames (512 bits
> for 2mb frames). Of course, we could add a way to communicate that we
> are using a different bitmap-granularity.

Yeah, please read the patches.  If they're not clear, then the
descriptions need work, but this is done already.