Re: [PATCH v2 0/5] mm / virtio: Provide support for page hinting

From: Michael S. Tsirkin
Date: Wed Jul 24 2019 - 17:32:19 EST

On Wed, Jul 24, 2019 at 09:47:24PM +0200, David Hildenbrand wrote:
> On 24.07.19 21:31, Michael S. Tsirkin wrote:
> > On Wed, Jul 24, 2019 at 08:41:33PM +0200, David Hildenbrand wrote:
> >> On 24.07.19 20:40, Nitesh Narayan Lal wrote:
> >>>
> >>> On 7/24/19 12:54 PM, Alexander Duyck wrote:
> >>>> This series provides an asynchronous means of hinting 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 page hinting
> >>>>
> >>>> The functionality for this is fairly simple. When enabled it will allocate
> >>>> statistics to track the number of hinted pages in a given free area. When
> >>>> the number of free pages exceeds this value plus a high water value,
> >>>> currently 32,
> >>> Shouldn't we configure this to a lower number such as 16?
> >>>> it will begin performing page hinting which consists of
> >>>> pulling pages off of free list and placing them into a scatter list. The
> >>>> scatterlist is then given to the page hinting device and it will perform
> >>>> the required action to make the pages "hinted", in the case of
> >>>> virtio-balloon this results in the pages being madvised as MADV_DONTNEED
> >>>> and as such they are forced out of the guest. After this they are placed
> >>>> back on the free list, and an additional bit is added if they are not
> >>>> merged indicating that they are a hinted buddy page instead of a standard
> >>>> buddy page. The cycle then repeats with additional non-hinted pages being
> >>>> pulled until the free areas all consist of hinted pages.
> >>>>
> >>>> I am leaving a number of things hard-coded such as limiting the lowest
> >>>> order processed to PAGEBLOCK_ORDER,
> >>> Have you considered making this option configurable at the compile time?
> >>>> and have left it up to the guest to
> >>>> determine what the limit is on how many pages it wants to allocate to
> >>>> process the hints.
> >>> It might make sense to set the number of pages to be hinted at a time from the
> >>> hypervisor.
> >>>>
> >>>> My primary testing has just been to verify the memory is being freed after
> >>>> allocation by running memhog 79g on a 80g guest and watching the total
> >>>> free memory via /proc/meminfo on the host. With this I have verified most
> >>>> of the memory is freed after each iteration. As far as performance I have
> >>>> been mainly focusing on the will-it-scale/page_fault1 test running with
> >>>> 16 vcpus. With that I have seen at most a 2% difference between the base
> >>>> kernel without these patches and the patches with virtio-balloon disabled.
> >>>> With the patches and virtio-balloon enabled with hinting the results
> >>>> largely depend on the host kernel. On a 3.10 RHEL kernel I saw up to a 2%
> >>>> drop in performance as I approached 16 threads,
> >>> I think this is acceptable.
> >>>> however on the the lastest
> >>>> linux-next kernel I saw roughly a 4% to 5% improvement in performance for
> >>>> all tests with 8 or more threads.
> >>> Do you mean that with your patches the will-it-scale/page_fault1 numbers were
> >>> better by 4-5% over an unmodified kernel?
> >>>> I believe the difference seen is due to
> >>>> the overhead for faulting pages back into the guest and zeroing of memory.
> >>> It may also make sense to test these patches with netperf to observe how much
> >>> performance drop it is introducing.
> >>>> Patch 4 is a bit on the large side at about 600 lines of change, however
> >>>> I really didn't see a good way to break it up since each piece feeds into
> >>>> the next. So I couldn't add the statistics by themselves as it didn't
> >>>> really make sense to add them without something that will either read or
> >>>> increment/decrement them, or add the Hinted state without something that
> >>>> would set/unset it. As such I just ended up adding the entire thing as
> >>>> one patch. It makes it a bit bigger but avoids the issues in the previous
> >>>> set where I was referencing things before they had been added.
> >>>>
> >>>> Changes from the RFC:
> >>>>
> >>>> Moved aeration requested flag out of aerator and into zone->flags.
> >>>> Moved bounary out of free_area and into local variables for aeration.
> >>>> Moved aeration cycle out of interrupt and into workqueue.
> >>>> Left nr_free as total pages instead of splitting it between raw and aerated.
> >>>> Combined size and physical address values in virtio ring into one 64b value.
> >>>>
> >>>> Changes from v1:
> >>>>
> >>>> Dropped "waste page treatment" in favor of "page hinting"
> >>> We may still have to try and find a better name for virtio-balloon side changes.
> >>> As "FREE_PAGE_HINT" and "PAGE_HINTING" are still confusing.
> >>
> >> We should have named that free page reporting, but that train already
> >> has left.
> >
> > I think VIRTIO_BALLOON_F_FREE_PAGE_HINT is different and arguably
> > actually does provide hints.
> I guess it depends on the point of view (e.g., getting all free pages
> feels more like a report). But I could also live with using the term
> reporting in this context.
> We could go ahead and name it all "page reporting", would also work for me.

So there are actually three differences between the machanisms:
1. VIRTIO_BALLOON_F_FREE_PAGE_HINT sometimes reports pages which might no
longer be on the free list (with subtle limitations which sometimes
still allow hypervisor to discard the pages)
2. VIRTIO_BALLOON_F_FREE_PAGE_HINT starts reporting when requested by
3. VIRTIO_BALLOON_F_FREE_PAGE_HINT is not incremental: each request
by host reports all free memory

By comparison, the proposed patches:
- always report only actually free pages
- report at a random time
- report incrementally

> --
> Thanks,
> David / dhildenb