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Subject: Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page reporting

On 02.10.19 02:55, Alexander Duyck wrote:
> On Tue, Oct 1, 2019 at 12:16 PM Nitesh Narayan Lal <nitesh@redhat.com> wrote:
>> On 10/1/19 12:21 PM, Alexander Duyck wrote:
>>> On Tue, 2019-10-01 at 17:35 +0200, David Hildenbrand wrote:
>>>> On 01.10.19 17:29, Alexander Duyck wrote:
>>>>> This series provides an asynchronous means of reporting to a hypervisor
>>>>> that a guest page is no longer in use and can have the data associated
>>>>> with it dropped. To do this I have implemented functionality that allows
>>>>> for what I am referring to as unused page reporting. The advantage of
>>>>> unused page reporting is that we can support a significant amount of
>>>>> memory over-commit with improved performance as we can avoid having to
>>>>> write/read memory from swap as the VM will instead actively participate
>>>>> in freeing unused memory so it doesn't have to be written.
>>>>> The functionality for this is fairly simple. When enabled it will allocate
>>>>> statistics to track the number of reported pages in a given free area.
>>>>> When the number of free pages exceeds this value plus a high water value,
>>>>> currently 32, it will begin performing page reporting which consists of
>>>>> pulling non-reported pages off of the free lists of a given zone and
>>>>> placing them into a scatterlist. The scatterlist is then given to the page
>>>>> reporting device and it will perform the required action to make the pages
>>>>> "reported", in the case of virtio-balloon this results in the pages being
>>>>> madvised as MADV_DONTNEED. After this they are placed back on their
>>>>> original free list. If they are not merged in freeing an additional bit is
>>>>> set indicating that they are a "reported" buddy page instead of a standard
>>>>> buddy page. The cycle then repeats with additional non-reported pages
>>>>> being pulled until the free areas all consist of reported pages.
>>>>> In order to try and keep the time needed to find a non-reported page to
>>>>> a minimum we maintain a "reported_boundary" pointer. This pointer is used
>>>>> by the get_unreported_pages iterator to determine at what point it should
>>>>> resume searching for non-reported pages. In order to guarantee pages do
>>>>> not get past the scan I have modified add_to_free_list_tail so that it
>>>>> will not insert pages behind the reported_boundary. Doing this allows us
>>>>> to keep the overhead to a minimum as re-walking the list without the
>>>>> boundary will result in as much as 18% additional overhead on a 32G VM.
>>> <snip>
>>>>> As far as possible regressions I have focused on cases where performing
>>>>> the hinting would be non-optimal, such as cases where the code isn't
>>>>> needed as memory is not over-committed, or the functionality is not in
>>>>> use. I have been using the will-it-scale/page_fault1 test running with 16
>>>>> vcpus and have modified it to use Transparent Huge Pages. With this I see
>>>>> almost no difference with the patches applied and the feature disabled.
>>>>> Likewise I see almost no difference with the feature enabled, but the
>>>>> madvise disabled in the hypervisor due to a device being assigned. With
>>>>> the feature fully enabled in both guest and hypervisor I see a regression
>>>>> between -1.86% and -8.84% versus the baseline. I found that most of the
>>>>> overhead was due to the page faulting/zeroing that comes as a result of
>>>>> the pages having been evicted from the guest.
>>>> I think Michal asked for a performance comparison against Nitesh's
>>>> approach, to evaluate if keeping the reported state + tracking inside
>>>> the buddy is really worth it. Do you have any such numbers already? (or
>>>> did my tired eyes miss them in this cover letter? :/)
>>> I thought what Michal was asking for was what was the benefit of using the
>>> boundary pointer. I added a bit up above and to the description for patch
>>> 3 as on a 32G VM it adds up to about a 18% difference without factoring in
>>> the page faulting and zeroing logic that occurs when we actually do the
>>> madvise.
>>> Do we have a working patch set for Nitesh's code? The last time I tried
>>> running his patch set I ran into issues with kernel panics. If we have a
>>> known working/stable patch set I can give it a try.
>> Did you try the v12 patch-set [1]?
>> I remember that you reported the CPU stall issue, which I fixed in the v12.
>> [1] https://lkml.org/lkml/2019/8/12/593
> So I tried testing with the spin_lock calls replaced with spin_lock
> _irq to resolve the IRQ issue. I also had shuffle enabled in order to
> increase the number of pages being dirtied.
> With that setup the bitmap approach is running significantly worse
> then my approach, even with the boundary removed. Since I had to

It would make sense to share the setup+benchmark+performance indication
that you measured. You don't have to share the actual numbers.

> modify the code to even getting working I am not comfortable posting
> numbers. My suggestion would be to look at reworking the patch set and
> post numbers for my patch set versus the bitmap approach and we can
> look at them then. I would prefer not to spend my time fixing and
> tuning a patch set that I am still not convinced is viable.

I agree, I think Nitesh should work on his patch set and try to
reproduce what you are seeing.

Also, I think to make a precise statement of "which overhead comes with
external tracking", Nitesh should switch to an approach (motivated by
Michal) like

1. Sense lockless if a page is still free
2. start_isolate_page_range()
-> Failed? Skip
3. test_pages_isolated()
-> No? undo_isolate_page_range(), skip
4. Repeat for multiple pages + report
5. undo_isolate_page_range()

That is the bare minimum any external tracking will need = some overhead
for the tracking data. As a nice side effect, it get's rid of taking the
zone lock manually AFAIKS.

But that's unrelated to your series, only to quantify "how much" does
external tracking actually cost.



David / dhildenb

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