On Tue, Mar 12, 2019 at 2:53 PM David Hildenbrand <david@xxxxxxxxxx> wrote: > > On 12.03.19 22:13, 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. 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. 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. > > > > 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. > > > > 3. Currently we are still doing a large amount of processing in the > > page free path. Ideally we should look at getting away from trying to > > do so much per-cpu work and instead just have some small tasks that > > put the data needed in the page, and then have a separate thread > > walking the free_list checking that data, isolating the pages, hinting > > them, and then returning them back to the free_list. > > This is highly debatable. Whenever the is concurrency, there is the need > for locking (well, at least synchronization - maybe using existing locks > like the zone lock). The other thread has to run somewhere. One thread > per VCPU might not what we want ... sorting this out might be more > complicated than it would seem. I would suggest to defer the discussion > of this change to a later stage. It can be easily reworked later - in > theory :) I'm not suggesting anything too complex for now. I would be happy with just using the zone lock. The only other thing we would really need to make it work is some sort of bit we could set once a page has been hinted, and cleared when it is allocated. I"m leaning toward PG_owner_priv_1 at this point since it doesn't seem to be used in the buddy allocator but is heavily used/re-purposed in multiple other spots. > 1 and 2 you mention are the lower hanging fruits that will definitely > improve performance. Agreed. Although the challenge with 2 is getting to the page later instead of trying to immediately hint on the page we just freed. That is why I still thing 3 is going to tie in closely with 2. > -- > > Thanks, > > David / dhildenb
