On 13.03.19 17:37, Alexander Duyck wrote: > On Wed, Mar 13, 2019 at 5:18 AM David Hildenbrand <david@xxxxxxxxxx> wrote: >> >> On 13.03.19 12:54, Nitesh Narayan Lal wrote: >>> >>> On 3/12/19 5:13 PM, Alexander Duyck wrote: >>>> On Tue, Mar 12, 2019 at 12:46 PM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: >>>>> On 3/8/19 4:39 PM, Alexander Duyck wrote: >>>>>> On Fri, Mar 8, 2019 at 11:39 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: >>>>>>> On 3/8/19 2:25 PM, Alexander Duyck wrote: >>>>>>>> On Fri, Mar 8, 2019 at 11:10 AM Nitesh Narayan Lal <nitesh@xxxxxxxxxx> wrote: >>>>>>>>> On 3/8/19 1:06 PM, Alexander Duyck wrote: >>>>>>>>>> On Thu, Mar 7, 2019 at 6:32 PM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: >>>>>>>>>>> On Thu, Mar 07, 2019 at 02:35:53PM -0800, Alexander Duyck wrote: >>>>>>>>>>>> The only other thing I still want to try and see if I can do is to add >>>>>>>>>>>> a jiffies value to the page private data in the case of the buddy >>>>>>>>>>>> pages. >>>>>>>>>>> Actually there's one extra thing I think we should do, and that is make >>>>>>>>>>> sure we do not leave less than X% off the free memory at a time. >>>>>>>>>>> This way chances of triggering an OOM are lower. >>>>>>>>>> If nothing else we could probably look at doing a watermark of some >>>>>>>>>> sort so we have to have X amount of memory free but not hinted before >>>>>>>>>> we will start providing the hints. It would just be a matter of >>>>>>>>>> tracking how much memory we have hinted on versus the amount of memory >>>>>>>>>> that has been pulled from that pool. >>>>>>>>> This is to avoid false OOM in the guest? >>>>>>>> Partially, though it would still be possible. Basically it would just >>>>>>>> be a way of determining when we have hinted "enough". Basically it >>>>>>>> doesn't do us much good to be hinting on free memory if the guest is >>>>>>>> already constrained and just going to reallocate the memory shortly >>>>>>>> after we hinted on it. The idea is with a watermark we can avoid >>>>>>>> hinting until we start having pages that are actually going to stay >>>>>>>> free for a while. >>>>>>>> >>>>>>>>>> It is another reason why we >>>>>>>>>> probably want a bit in the buddy pages somewhere to indicate if a page >>>>>>>>>> has been hinted or not as we can then use that to determine if we have >>>>>>>>>> to account for it in the statistics. >>>>>>>>> The one benefit which I can see of having an explicit bit is that it >>>>>>>>> will help us to have a single hook away from the hot path within buddy >>>>>>>>> merging code (just like your arch_merge_page) and still avoid duplicate >>>>>>>>> hints while releasing pages. >>>>>>>>> >>>>>>>>> I still have to check PG_idle and PG_young which you mentioned but I >>>>>>>>> don't think we can reuse any existing bits. >>>>>>>> Those are bits that are already there for 64b. I think those exist in >>>>>>>> the page extension for 32b systems. If I am not mistaken they are only >>>>>>>> used in VMA mapped memory. What I was getting at is that those are the >>>>>>>> bits we could think about reusing. >>>>>>>> >>>>>>>>> If we really want to have something like a watermark, then can't we use >>>>>>>>> zone->free_pages before isolating to see how many free pages are there >>>>>>>>> and put a threshold on it? (__isolate_free_page() does a similar thing >>>>>>>>> but it does that on per request basis). >>>>>>>> Right. That is only part of it though since that tells you how many >>>>>>>> free pages are there. But how many of those free pages are hinted? >>>>>>>> That is the part we would need to track separately and then then >>>>>>>> compare to free_pages to determine if we need to start hinting on more >>>>>>>> memory or not. >>>>>>> Only pages which are isolated will be hinted, and once a page is >>>>>>> isolated it will not be counted in the zone free pages. >>>>>>> Feel free to correct me if I am wrong. >>>>>> You are correct up to here. When we isolate the page it isn't counted >>>>>> against the free pages. However after we complete the hint we end up >>>>>> taking it out of isolation and returning it to the "free" state, so it >>>>>> will be counted against the free pages. >>>>>> >>>>>>> If I am understanding it correctly you only want to hint the idle pages, >>>>>>> is that right? >>>>>> Getting back to the ideas from our earlier discussion, we had 3 stages >>>>>> for things. Free but not hinted, isolated due to hinting, and free and >>>>>> hinted. So what we would need to do is identify the size of the first >>>>>> pool that is free and not hinted by knowing the total number of free >>>>>> pages, and then subtract the size of the pages that are hinted and >>>>>> still free. >>>>> To summarize, for now, I think it makes sense to stick with the current >>>>> approach as this way we can avoid any locking in the allocation path and >>>>> reduce the number of hypercalls for a bunch of MAX_ORDER - 1 page. >>>> I'm not sure what you are talking about by "avoid any locking in the >>>> allocation path". Are you talking about the spin on idle bit, if so >>>> then yes. >>> Yeap! >>>> However I have been testing your patches and I was correct >>>> in the assumption that you forgot to handle the zone lock when you >>>> were freeing __free_one_page. >>> Yes, these are the steps other than the comments you provided in the >>> code. (One of them is to fix release_buddy_page()) >>>> I just did a quick copy/paste from your >>>> zone lock handling from the guest_free_page_hinting function into the >>>> release_buddy_pages function and then I was able to enable multiple >>>> CPUs without any issues. >>>> >>>>> For the next step other than the comments received in the code and what >>>>> I mentioned in the cover email, I would like to do the following: >>>>> 1. Explore the watermark idea suggested by Alex and bring down memhog >>>>> execution time if possible. >>>> So there are a few things that are hurting us on the memhog test: >>>> 1. The current QEMU patch is only madvising 4K pages at a time, this >>>> is disabling THP and hurts the test. >>> Makes sense, thanks for pointing this out. >>>> >>>> 2. The fact that we madvise the pages away makes it so that we have to >>>> fault the page back in in order to use it for the memhog test. In >>>> order to avoid that penalty we may want to see if we can introduce >>>> some sort of "timeout" on the pages so that we are only hinting away >>>> old pages that have not been used for some period of time. >>> >>> Possibly using MADVISE_FREE should also help in this, I will try this as >>> well. >> >> I was asking myself some time ago how MADVISE_FREE will be handled in >> case of THP. Please let me know your findings :) > > The problem with MADVISE_FREE is that it will add additional > complication to the QEMU portion of all this as it only applies to > anonymous memory if I am not mistaken. Just as MADV_DONTNEED. So nothing new. Future work. -- Thanks, David / dhildenb
