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

[Alexander Duyck]

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.

- Alex