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SubjectRe: [PATCH v28 0/4] Virtio-balloon: support free page reporting
On Thu, Feb 08, 2018 at 05:50:16PM +0800, Wei Wang wrote:
> This patch series is separated from the previous "Virtio-balloon
> Enhancement" series. The new feature, VIRTIO_BALLOON_F_FREE_PAGE_HINT,
> implemented by this series enables the virtio-balloon driver to report
> hints of guest free pages to the host. It can be used to accelerate live
> migration of VMs. Here is an introduction of this usage:
> Live migration needs to transfer the VM's memory from the source machine
> to the destination round by round. For the 1st round, all the VM's memory
> is transferred. From the 2nd round, only the pieces of memory that were
> written by the guest (after the 1st round) are transferred. One method
> that is popularly used by the hypervisor to track which part of memory is
> written is to write-protect all the guest memory.
> This feature enables the optimization of the 1st round memory transfer -
> the hypervisor can skip the transfer of guest free pages in the 1st round.
> It is not concerned that the memory pages are used after they are given
> to the hypervisor as a hint of the free pages, because they will be
> tracked by the hypervisor and transferred in the next round if they are
> used and written.

At this point I think it's a good idea to focus on the qemu
side, that code is still not really ready yet.

> * Tests
> - Migration time improvement
> Result:
> Live migration time is reduced to 14% with this optimization.
> Details:
> Local live migration of 8GB idle guest, the legacy live migration takes
> ~1817ms. With this optimization, it takes ~254ms, which reduces the time
> to 14%.

> - Workload tests
> Results:
> Running this feature has no impact on the linux compilation workload
> running inside the guest.

I think you should try something memory intensive. Try asking
qemu migration guys for hints on a good test to run.

> Details:
> Set up a Ping-Pong local live migration, where the guest ceaselessy
> migrates between the source and destination. Linux compilation,
> i.e. make bzImage -j4, is performed during the Ping-Pong migration. The
> legacy case takes 5min14s to finish the compilation. With this
> optimization patched, it takes 5min12s.

How is migration time affected in this case?

> ChangeLog:
> v27->v28:
> - mm/page_poison: Move PAGE_POISON to page_poison.c and add a function
> to expose page poison val to kernel modules.
> v26->v27:
> - add a new patch to expose page_poisoning_enabled to kernel modules
> - virtio-balloon: set poison_val to 0xaaaaaaaa, instead of 0xaa
> v25->v26: virtio-balloon changes only
> - remove kicking free page vq since the host now polls the vq after
> initiating the reporting
> - report_free_page_func: detach all the used buffers after sending
> the stop cmd id. This avoids leaving the detaching burden (i.e.
> overhead) to the next cmd id. Detaching here isn't considered
> overhead since the stop cmd id has been sent, and host has already
> moved formard.
> v24->v25:
> - mm: change walk_free_mem_block to return 0 (instead of true) on
> completing the report, and return a non-zero value from the
> callabck, which stops the reporting.
> - virtio-balloon:
> - use enum instead of define for VIRTIO_BALLOON_VQ_INFLATE etc.
> - avoid __virtio_clear_bit when bailing out;
> - a new method to avoid reporting the some cmd id to host twice
> - destroy_workqueue can cancel free page work when the feature is
> negotiated;
> - fail probe when the free page vq size is less than 2.
> v23->v24:
> - change feature name VIRTIO_BALLOON_F_FREE_PAGE_VQ to
> - kick when vq->num_free < half full, instead of "= half full"
> - replace BUG_ON with bailing out
> - check vb->balloon_wq in probe(), if null, bail out
> - add a new feature bit for page poisoning
> - solve the corner case that one cmd id being sent to host twice
> v22->v23:
> - change to kick the device when the vq is half-way full;
> - open-code batch_free_page_sg into add_one_sg;
> - change cmd_id from "uint32_t" to "__virtio32";
> - reserver one entry in the vq for the driver to send cmd_id, instead
> of busywaiting for an available entry;
> - add "stop_update" check before queue_work for prudence purpose for
> now, will have a separate patch to discuss this flag check later;
> - init_vqs: change to put some variables on stack to have simpler
> implementation;
> - add destroy_workqueue(vb->balloon_wq);
> v21->v22:
> - add_one_sg: some code and comment re-arrangement
> - send_cmd_id: handle a cornercase
> For previous ChangeLog, please reference
> Wei Wang (4):
> mm: support reporting free page blocks
> mm/page_poison: add a function to expose page poison val to kernel
> modules
> drivers/virtio/virtio_balloon.c | 258 +++++++++++++++++++++++++++++++-----
> include/linux/mm.h | 8 ++
> include/linux/poison.h | 7 -
> include/uapi/linux/virtio_balloon.h | 7 +
> mm/page_alloc.c | 96 ++++++++++++++
> mm/page_poison.c | 24 ++++
> 6 files changed, 357 insertions(+), 43 deletions(-)
> --
> 2.7.4

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