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: >>>>> 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. >> 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. -- Thanks Nitesh
