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

From: Nitesh Narayan Lal
Date: Wed Jul 24 2019 - 16:15:19 EST

On 7/24/19 3:47 PM, 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.
I think that should work.
Having two separate names one for the kernel and the other for the virtio
interface will cause unnecessary confusion.