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 17:21:53 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Dave Hansen <dave.hansen@...el.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

>>>
>>> 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.

So we can send length in powers of two. Still, I don't see any benefit
over a simple pfn/len schema. But I'll have a more detailed look at the
implementation first, maybe that will enlighten me :)

>
>> 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.
>

I missed the page_shift, thanks for the hint.

-- 

David