On Thu, Jul 25, 2019 at 09:16:21AM -0700, 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. 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. Good points. > > 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. As a result you could end up walking the bitmap for a while > clearing bits without ever finding a free page. For sure. > > 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
