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. 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. 2. Benchmark hinting v/s non-hinting more extensively. Let me know if you have any specific suggestions in terms of the tools I can run to do the same. (I am planning to run atleast netperf, hackbench and stress for this). > >>>>>>> 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 >> -- Regards Nitesh
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