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

[David Hildenbrand <david@redhat.com>]

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@localhost.localdomain/
>> 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@localhost.localdomain/
>> 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.

-- 

Thanks,

David / dhildenb