On 31.03.20 16:29, David Hildenbrand wrote: > On 31.03.20 16:18, Michael S. Tsirkin wrote: >> On Tue, Mar 31, 2020 at 04:09:59PM +0200, David Hildenbrand wrote: >> >> ... >> >>>>>>>>>>>>>> So if we want to address this, IMHO this calls for a new API. >>>>>>>>>>>>>> Along the lines of >>>>>>>>>>>>>> >>>>>>>>>>>>>> struct page *alloc_page_range(gfp_t gfp, unsigned int min_order, >>>>>>>>>>>>>> unsigned int max_order, unsigned int *order) >>>>>>>>>>>>>> >>>>>>>>>>>>>> the idea would then be to return at a number of pages in the given >>>>>>>>>>>>>> range. >>>>>>>>>>>>>> >>>>>>>>>>>>>> What do you think? Want to try implementing that? >> >> .. >> >>> I expect the whole "steal huge pages from your guest" to be problematic, >>> as I already mentioned to Alex. This needs a performance evaluation. >>> >>> This all smells like a lot of workload dependent fine-tuning. :) >> >> >> So that's why I proposed the API above. >> >> The idea is that *if we are allocating a huge page anyway*, >> rather than break it up let's send it whole to the device. >> If we have smaller pages, return smaller pages. >> > > Sorry, I still fail to see why you cannot do that with my version of > balloon_pages_alloc(). But maybe I haven't understood the magic you > expect to happen in alloc_page_range() :) > > It's just going via a different inflate queue once we have that page, as > I stated in front of my draft patch "but with an > optimized reporting interface". > >> That seems like it would always be an improvement, whatever the >> workload. >> > > Don't think so. Assume there are plenty of 4k pages lying around. It > might actually be *bad* for guest performance if you take a huge page > instead of all the leftover 4k pages that cannot be merged. Only at the > point where you would want to break a bigger page up and report it in > pieces, where it would definitely make no difference. I just understood what you mean :) and now it makes sense - it avoids exactly that. Basically 1. Try to allocate order-0. No split necessary? return the page 2. Try to allocate order-1. No split necessary? return the page ... up to MAX_ORDER - 1. Yeah, I guess this will need a new kernel API. -- Thanks, David / dhildenb _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
