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. > -- > > Thanks, > > David / dhildenb
