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

[David Hildenbrand]

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:
>>>> https://lore.kernel.org/lkml/20190530215223.13974.22445.stgit@xxxxxxxxxxxxxxxxxxxxx/
>>>> 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:
>>>> https://lore.kernel.org/lkml/20190619222922.1231.27432.stgit@xxxxxxxxxxxxxxxxxxxxx/
>>>> 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.

-- 

Thanks,

David / dhildenb