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

Nitesh Narayan Lal
2019-10-01T19:16:19Z

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

>
> - Alex
>
--
Thanks
Nitesh