Linux Virtualization - Jun 2018 - [PATCH v34 0/4] Virtio-balloon: support free page reporting

On 06/27/2018 07:06 PM, David Hildenbrand wrote:
> On 25.06.2018 14:05, 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 by skipping the transfer of guest
>> free pages during VM live migration. 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 subsequent round if they are used and written.
>>
>> * Tests
>> - Test Environment
>>      Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
>>      Guest: 8G RAM, 4 vCPU
>>      Migration setup: migrate_set_speed 100G, migrate_set_downtime 2
second
>>
>> - Test Results
>>      - Idle Guest Live Migration Time (results are averaged over 10
runs):
>>          - Optimization v.s. Legacy = 284ms vs 1757ms --> ~84%
reduction
>>      - Guest with Linux Compilation Workload (make bzImage -j4):
>>          - Live Migration Time (average)
>>            Optimization v.s. Legacy = 1402ms v.s. 2528ms --> ~44%
reduction
>>          - Linux Compilation Time
>>            Optimization v.s. Legacy = 5min6s v.s. 5min12s
>>            --> no obvious difference
>>
> Being in version 34 already, this whole thing still looks and feels like
> a big hack to me. It might just be me, but especially if I read about
> assumptions like "QEMU will not hotplug memory during migration".
This
> does not feel like a clean solution.
>
> I am still not sure if we really need this interface, especially as real
> free page hinting might be on its way.
>
> a) we perform free page hinting by setting all free pages
> (arch_free_page()) to zero. Migration will detect zero pages and
> minimize #pages to migrate. I don't think this is a good idea but
Michel
> suggested to do a performance evaluation and Nitesh is looking into that
> right now.
The hypervisor doesn't get the zero pages for free. It pays lots of CPU 
utilization and memory bandwidth (there are some guys complaining abut 
this on the QEMU mailinglist)
In the above results, the legacy part already has the zero page feature 
in use.
>
> b) we perform free page hinting using something that Nitesh proposed. We
> get in QEMU blocks of free pages that we can MADV_FREE. In addition we
> could e.g. clear the dirty bit of these pages in the dirty bitmap, to
> hinder them from getting migrated. Right now the hinting mechanism is
> synchronous (called from arch_free_page()) but we might be able to
> convert it into something asynchronous.
>
> So we might be able to completely get rid of this interface. And looking
> at all the discussions and problems that already happened during the
> development of this series, I think we should rather look into how clean
> free page hinting might solve the same problem.
>
> If it can't be solved using free page hinting, fair enough.
>
I'm afraid it can't. For example, when we have a guest booted, without 
too many memory activities. Assume the guest has 8GB free memory. The 
arch_free_page there won't be able to capture the 8GB free pages since 
there is no free() called. This results in no free pages reported to host.

Best,
Wei

On 29.06.2018 05:51, Wei Wang wrote:
> On 06/27/2018 07:06 PM, David Hildenbrand wrote:
>> On 25.06.2018 14:05, 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 by skipping the transfer of
guest
>>> free pages during VM live migration. 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 subsequent round if they are used and written.
>>>
>>> * Tests
>>> - Test Environment
>>>      Host: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz
>>>      Guest: 8G RAM, 4 vCPU
>>>      Migration setup: migrate_set_speed 100G, migrate_set_downtime
2 second
>>>
>>> - Test Results
>>>      - Idle Guest Live Migration Time (results are averaged over 10
runs):
>>>          - Optimization v.s. Legacy = 284ms vs 1757ms --> ~84%
reduction
>>>      - Guest with Linux Compilation Workload (make bzImage -j4):
>>>          - Live Migration Time (average)
>>>            Optimization v.s. Legacy = 1402ms v.s. 2528ms -->
~44% reduction
>>>          - Linux Compilation Time
>>>            Optimization v.s. Legacy = 5min6s v.s. 5min12s
>>>            --> no obvious difference
>>>
>> Being in version 34 already, this whole thing still looks and feels
like
>> a big hack to me. It might just be me, but especially if I read about
>> assumptions like "QEMU will not hotplug memory during
migration". This
>> does not feel like a clean solution.
>>
>> I am still not sure if we really need this interface, especially as
real
>> free page hinting might be on its way.
>>
>> a) we perform free page hinting by setting all free pages
>> (arch_free_page()) to zero. Migration will detect zero pages and
>> minimize #pages to migrate. I don't think this is a good idea but
Michel
>> suggested to do a performance evaluation and Nitesh is looking into
that
>> right now.
> 
> The hypervisor doesn't get the zero pages for free. It pays lots of CPU
> utilization and memory bandwidth (there are some guys complaining abut 
> this on the QEMU mailinglist)
> In the above results, the legacy part already has the zero page feature 
> in use.
Indeed, I don't consider this attempt not very practical in general,
especially as it would rely on KSM right now, which is frowned upon by
many people.
> 
>>
>> b) we perform free page hinting using something that Nitesh proposed.
We
>> get in QEMU blocks of free pages that we can MADV_FREE. In addition we
>> could e.g. clear the dirty bit of these pages in the dirty bitmap, to
>> hinder them from getting migrated. Right now the hinting mechanism is
>> synchronous (called from arch_free_page()) but we might be able to
>> convert it into something asynchronous.
>>
>> So we might be able to completely get rid of this interface. And
looking
>> at all the discussions and problems that already happened during the
>> development of this series, I think we should rather look into how
clean
>> free page hinting might solve the same problem.
>>
>> If it can't be solved using free page hinting, fair enough.
>>
> 
> I'm afraid it can't. For example, when we have a guest booted,
without
> too many memory activities. Assume the guest has 8GB free memory. The 
> arch_free_page there won't be able to capture the 8GB free pages since 
> there is no free() called. This results in no free pages reported to host.

So, it takes some time from when the guest boots up until the balloon
device was initialized and therefore page hinting can start. For that
period, you won't get any arch_free_page()/page hinting callbacks, correct.

However in the hypervisor, you can theoretically track which pages the
guest actually touched ("dirty"), so you already know "which
pages were
never touched while booting up until virtio-balloon was brought to
life". These, you can directly exclude from migration. No interface
required.

The remaining problem is pages that were touched ("allocated") by the
guest during bootup but freed again, before virtio-balloon came up. One
would have to measure how many pages these usually are, I would say it
would not be that many (because recently freed pages are likely to be
used again next for allocation). However, there are some pages not being
reported.

During the lifetime of the guest, this should not be a problem,
eventually one of these pages would get allocated/freed again, so the
problem "solves itself over time". You are looking into the special
case
of migrating the VM just after it has been started. But we have the
exact same problem also for ordinary free page hinting, so we should
rather solve that problem. It is not migration specific.

If we are looking for an alternative to "problem solves itself",
something like "if virtio-balloon comes up, it will report all free
pages step by step using free page hinting, just like we would have from
"arch_free_pages()"". This would be the same interface we are
using for
free page hinting - and it could even be made configurable in the guest.

The current approach we are discussing internally for details about
Nitesh's work ("how the magic inside arch_fee_pages() will work
efficiently) would allow this as far as I can see just fine.

There would be a tiny little window between virtio-balloon comes up and
it has reported all free pages step by step, but that can be considered
a very special corner case that I would argue is not worth it to be
optimized.

If I am missing something important here, sorry in advance :)
> 
> Best,
> Wei
> 

-- 

Thanks,

David / dhildenb

On 06/29/2018 03:46 PM, David Hildenbrand wrote:
>>
>> I'm afraid it can't. For example, when we have a guest booted,
without
>> too many memory activities. Assume the guest has 8GB free memory. The
>> arch_free_page there won't be able to capture the 8GB free pages
since
>> there is no free() called. This results in no free pages reported to
host.
>
> So, it takes some time from when the guest boots up until the balloon
> device was initialized and therefore page hinting can start. For that
> period, you won't get any arch_free_page()/page hinting callbacks,
correct.
>
> However in the hypervisor, you can theoretically track which pages the
> guest actually touched ("dirty"), so you already know "which
pages were
> never touched while booting up until virtio-balloon was brought to
> life". These, you can directly exclude from migration. No interface
> required.
>
> The remaining problem is pages that were touched ("allocated") by
the
> guest during bootup but freed again, before virtio-balloon came up. One
> would have to measure how many pages these usually are, I would say it
> would not be that many (because recently freed pages are likely to be
> used again next for allocation). However, there are some pages not being
> reported.
>
> During the lifetime of the guest, this should not be a problem,
> eventually one of these pages would get allocated/freed again, so the
> problem "solves itself over time". You are looking into the
special case
> of migrating the VM just after it has been started. But we have the
> exact same problem also for ordinary free page hinting, so we should
> rather solve that problem. It is not migration specific.
>
> If we are looking for an alternative to "problem solves itself",
> something like "if virtio-balloon comes up, it will report all free
> pages step by step using free page hinting, just like we would have from
> "arch_free_pages()"". This would be the same interface we
are using for
> free page hinting - and it could even be made configurable in the guest.
>
> The current approach we are discussing internally for details about
> Nitesh's work ("how the magic inside arch_fee_pages() will work
> efficiently) would allow this as far as I can see just fine.
>
> There would be a tiny little window between virtio-balloon comes up and
> it has reported all free pages step by step, but that can be considered
> a very special corner case that I would argue is not worth it to be
> optimized.
>
> If I am missing something important here, sorry in advance :)
>
Probably I didn't explain that well. Please see my re-try:

That work is to monitor page allocation and free activities via 
arch_alloc_pages and arch_free_pages. It has per-CPU lists to record the 
pages that are freed to the mm free list, and the per-CPU lists dump the 
recorded pages to a global list when any of them is full.
So its own per-CPU list will only be able to get free pages when there 
is an mm free() function gets called. If we have 8GB free memory on the 
mm free list, but no application uses them and thus no mm free() calls 
are made. In that case, the arch_free_pages isn't called, and no free 
pages added to the per-CPU list, but we have 8G free memory right on the 
mm free list.
How would you guarantee the per-CPU lists have got all the free pages 
that the mm free lists have?

- I'm also worried about the overhead of maintaining so many per-CPU 
lists and the global list. For example, if we have applications 
frequently allocate and free 4KB pages, and each per-CPU list needs to 
implement the buddy algorithm to sort and merge neighbor pages. Today a 
server can have more than 100 CPUs, then there will be more than 100 
per-CPU lists which need to sync to a global list under a lock, I'm not 
sure if this would scale well.

- This seems to be a burden imposed on the core mm memory 
allocation/free path. The whole overhead needs to be carried during the 
whole system life cycle. What we actually expected is to just make one 
call to get the free page hints only when live migration happens.

Best,
Wei