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Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimiz

From: Roman Kagan
Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization
Date: Thu, 3 Mar 2016 16:58:34 +0300
User-agent: Mutt/1.5.24 (2015-08-30)

On Thu, Mar 03, 2016 at 06:44:24PM +0800, Liang Li wrote:
> The current QEMU live migration implementation mark the all the
> guest's RAM pages as dirtied in the ram bulk stage, all these pages
> will be processed and that takes quit a lot of CPU cycles.
> From guest's point of view, it doesn't care about the content in free
> pages. We can make use of this fact and skip processing the free
> pages in the ram bulk stage, it can save a lot CPU cycles and reduce
> the network traffic significantly while speed up the live migration
> process obviously.
> This patch set is the QEMU side implementation.
> The virtio-balloon is extended so that QEMU can get the free pages
> information from the guest through virtio.
> After getting the free pages information (a bitmap), QEMU can use it
> to filter out the guest's free pages in the ram bulk stage. This make
> the live migration process much more efficient.
> This RFC version doesn't take the post-copy and RDMA into
> consideration, maybe both of them can benefit from this PV solution
> by with some extra modifications.
> Performance data
> ================
> Test environment:
> CPU: Intel (R) Xeon(R) CPU ES-2699 v3 @ 2.30GHz
> Host RAM: 64GB
> Host Linux Kernel:  4.2.0           Host OS: CentOS 7.1
> Guest Linux Kernel:  4.5.rc6        Guest OS: CentOS 6.6
> Network:  X540-AT2 with 10 Gigabit connection
> Guest RAM: 8GB
> Case 1: Idle guest just boots:
> ============================================
>                     | original  |    pv    
> -------------------------------------------
> total time(ms)      |    1894   |   421
> --------------------------------------------
> transferred ram(KB) |   398017  |  353242
> ============================================
> Case 2: The guest has ever run some memory consuming workload, the
> workload is terminated just before live migration.
> ============================================
>                     | original  |    pv    
> -------------------------------------------
> total time(ms)      |   7436    |   552
> --------------------------------------------
> transferred ram(KB) |  8146291  |  361375
> ============================================

Both cases look very artificial to me.  Normally you migrate VMs which
have started long ago and which can't have their services terminated
before the migration, so I wouldn't expect any useful amount of free
pages obtained this way.

OTOH I don't see why you can't just inflate the balloon before the
migration, and really optimize the amount of transferred data this way?
With the recently proposed VIRTIO_BALLOON_S_AVAIL you can have a fairly
good estimate of the optimal balloon size, and with the recently merged
balloon deflation on OOM it's a safe thing to do without exposing the
guest workloads to OOM risks.


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