On Thu, Feb 07, 2019 at 09:43:44AM -0800, Alexander Duyck wrote: > On Tue, Feb 5, 2019 at 3:21 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > On Tue, Feb 05, 2019 at 04:54:03PM -0500, Nitesh Narayan Lal wrote: > > > > > > On 2/5/19 3:45 PM, Michael S. Tsirkin wrote: > > > > On Mon, Feb 04, 2019 at 03:18:53PM -0500, Nitesh Narayan Lal wrote: > > > >> This patch enables the kernel to scan the per cpu array and > > > >> compress it by removing the repetitive/re-allocated pages. > > > >> Once the per cpu array is completely filled with pages in the > > > >> buddy it wakes up the kernel per cpu thread which re-scans the > > > >> entire per cpu array by acquiring a zone lock corresponding to > > > >> the page which is being scanned. If the page is still free and > > > >> present in the buddy it tries to isolate the page and adds it > > > >> to another per cpu array. > > > >> > > > >> Once this scanning process is complete and if there are any > > > >> isolated pages added to the new per cpu array kernel thread > > > >> invokes hyperlist_ready(). > > > >> > > > >> In hyperlist_ready() a hypercall is made to report these pages to > > > >> the host using the virtio-balloon framework. In order to do so > > > >> another virtqueue 'hinting_vq' is added to the balloon framework. > > > >> As the host frees all the reported pages, the kernel thread returns > > > >> them back to the buddy. > > > >> > > > >> Signed-off-by: Nitesh Narayan Lal <nitesh@xxxxxxxxxx> > > > > > > > > This looks kind of like what early iterations of Wei's patches did. > > > > > > > > But this has lots of issues, for example you might end up with > > > > a hypercall per a 4K page. > > > > So in the end, he switched over to just reporting only > > > > MAX_ORDER - 1 pages. > > > You mean that I should only capture/attempt to isolate pages with order > > > MAX_ORDER - 1? > > > > > > > > Would that be a good idea for you too? > > > Will it help if we have a threshold value based on the amount of memory > > > captured instead of the number of entries/pages in the array? > > > > This is what Wei's patches do at least. > > So in the solution I had posted I was looking more at > HUGETLB_PAGE_ORDER and above as the size of pages to provide the hints > on [1]. The advantage to doing that is that you can also avoid > fragmenting huge pages which in turn can cause what looks like a > memory leak as the memory subsystem attempts to reassemble huge > pages[2]. In my mind a 2MB page makes good sense in terms of the size > of things to be performing hints on as anything smaller than that is > going to just end up being a bunch of extra work and end up causing a > bunch of fragmentation. Yes MAX_ORDER-1 is 4M. So not a lot of difference on x86. The idea behind keying off MAX_ORDER is that CPU hugepages isn't the only reason to avoid fragmentation, there's other hardware that benefits from linear physical addresses. And there are weird platforms where HUGETLB_PAGE_ORDER exceeds MAX_ORDER - 1. So from that POV keying it off MAX_ORDER makes more sense. > The only issue with limiting things on an arbitrary boundary like that > is that you have to hook into the buddy allocator to catch the cases > where a page has been merged up into that range. > > [1] https://lkml.org/lkml/2019/2/4/903 > [2] https://blog.digitalocean.com/transparent-huge-pages-and-alternative-memory-allocators/
