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. If I am understanding it correctly you only want to hint the idle pages, is that right? > >>>>> With that we could track the age of the page so it becomes >>>>> easier to only target pages that are truly going cold rather than >>>>> trying to grab pages that were added to the freelist recently. >>>> I like that but I have a vague memory of discussing this with Rik van >>>> Riel and him saying it's actually better to take away recently used >>>> ones. Can't see why would that be but maybe I remember wrong. Rik - am I >>>> just confused? >>> It is probably to cut down on the need for disk writes in the case of >>> swap. If that is the case it ends up being a trade off. >>> >>> The sooner we hint the less likely it is that we will need to write a >>> given page to disk. However the sooner we hint, the more likely it is >>> we will need to trigger a page fault and pull back in a zero page to >>> populate the last page we were working on. The sweet spot will be that >>> period of time that is somewhere in between so we don't trigger >>> unnecessary page faults and we don't need to perform additional swap >>> reads/writes. >> -- >> Regards >> Nitesh >> -- Regards Nitesh
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