On 7/25/19 12:16 PM, Alexander Duyck wrote: > On Thu, 2019-07-25 at 11:16 -0400, Michael S. Tsirkin wrote: >> On Thu, Jul 25, 2019 at 08:05:30AM -0700, Alexander Duyck wrote: >>> On Thu, 2019-07-25 at 07:35 -0400, Nitesh Narayan Lal wrote: >>>> On 7/24/19 6:03 PM, Alexander Duyck wrote: >>>>> On Wed, 2019-07-24 at 17:38 -0400, Michael S. Tsirkin wrote: >>>>>> On Wed, Jul 24, 2019 at 10:12:10AM -0700, Alexander Duyck wrote: >>>>>>> From: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> >>>>>>> >>>>>>> Add support for what I am referring to as "bubble hinting". Basically the >>>>>>> idea is to function very similar to how the balloon works in that we >>>>>>> basically end up madvising the page as not being used. However we don't >>>>>>> really need to bother with any deflate type logic since the page will be >>>>>>> faulted back into the guest when it is read or written to. >>>>>>> >>>>>>> This is meant to be a simplification of the existing balloon interface >>>>>>> to use for providing hints to what memory needs to be freed. I am assuming >>>>>>> this is safe to do as the deflate logic does not actually appear to do very >>>>>>> much other than tracking what subpages have been released and which ones >>>>>>> haven't. >>>>>>> >>>>>>> Signed-off-by: Alexander Duyck <alexander.h.duyck@xxxxxxxxxxxxxxx> >>>>>> BTW I wonder about migration here. When we migrate we lose all hints >>>>>> right? Well destination could be smarter, detect that page is full of >>>>>> 0s and just map a zero page. Then we don't need a hint as such - but I >>>>>> don't think it's done like that ATM. >>>>> I was wondering about that a bit myself. If you migrate with a balloon >>>>> active what currently happens with the pages in the balloon? Do you >>>>> actually migrate them, or do you ignore them and just assume a zero page? >>>>> I'm just reusing the ram_block_discard_range logic that was being used for >>>>> the balloon inflation so I would assume the behavior would be the same. >>>> I agree, however, I think it is worth investigating to see if enabling hinting >>>> adds some sort of overhead specifically in this kind of scenarios. What do you >>>> think? >>> I suspect that the hinting/reporting would probably improve migration >>> times based on the fact that from the sound of things it would just be >>> migrated as a zero page. >>> >>> I don't have a good setup for testing migration though and I am not that >>> familiar with trying to do a live migration. That is one of the reasons >>> why I didn't want to stray too far from the existing balloon code as that >>> has already been tested with migration so I would assume as long as I am >>> doing almost the exact same thing to hint the pages away it should behave >>> exactly the same. >>> >>>>>> I also wonder about interaction with deflate. ATM deflate will add >>>>>> pages to the free list, then balloon will come right back and report >>>>>> them as free. >>>>> I don't know how likely it is that somebody who is getting the free page >>>>> reporting is likely to want to also use the balloon to take up memory. >>>> I think it is possible. There are two possibilities: >>>> 1. User has a workload running, which is allocating and freeing the pages and at >>>> the same time, user deflates. >>>> If these new pages get used by this workload, we don't have to worry as you are >>>> already handling that by not hinting the free pages immediately. >>>> 2. Guest is idle and the user adds up some memory, for this situation what you >>>> have explained below does seems reasonable. >>> Us hinting on pages that are freed up via deflate wouldn't be too big of a >>> deal. I would think that is something we could look at addressing as more >>> of a follow-on if we ever needed to since it would just add more >>> complexity. >>> >>> Really what I would like to see is the balloon itself get updated first to >>> perhaps work with variable sized pages first so that we could then have >>> pages come directly out of the balloon and go back into the freelist as >>> hinted, or visa-versa where hinted pages could be pulled directly into the >>> balloon without needing to notify the host. >> Right, I agree. At this point the main thing I worry about is that >> the interfaces only support one reporter, since a page flag is used. >> So if we ever rewrite existing hinting to use the new mm >> infrastructure then we can't e.g. enable both types of hinting. > Does it make sense to have multiple types of hinting active at the same > time though? That kind of seems wasteful to me. I agree. > Ideally we should be able > to provide the hints and have them feed whatever is supposed to be using > them. So for example I could probably look at also clearing the bitmaps > when migration is in process. > > Also, I am wonder if the free page hints would be redundant with the form > of page hinting/reporting that I have since we should be migrating a much > smaller footprint anyway if the pages have been madvised away before we > even start the migration. > >> FWIW Nitesh's RFC does not have this limitation. > Yes, but there are also limitations to his approach. For example the fact > that the bitmap it maintains is back to being a hint rather then being > very exact. True. > As a result you could end up walking the bitmap for a while > clearing bits without ever finding a free page. Are referring to the overhead which will be introduced due to bitmap scanning on very large guests? > >> I intend to think about this over the weekend. > Sounds good. I'll try to get the stuff you have pointed out so far > addressed and hopefully have v3 ready to go next week. > > Thanks. > > - Alex > -- Thanks Nitesh
