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. -- Thanks, David / dhildenb
