Messages in this thread | | | Date | Thu, 11 Nov 2021 20:32:39 +0800 | Subject | Re: [PATCH v3 00/15] Free user PTE page table pages | From | Qi Zheng <> |
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On 11/11/21 8:20 PM, David Hildenbrand wrote: > On 11.11.21 13:00, Qi Zheng wrote: >> >> >> On 11/11/21 7:19 PM, David Hildenbrand wrote: >>> On 11.11.21 12:08, Qi Zheng wrote: >>>> >>>> >>>> On 11/11/21 5:22 PM, David Hildenbrand wrote: >>>>> On 11.11.21 04:58, Qi Zheng wrote: >>>>>> >>>>>> >>>>>> On 11/11/21 1:37 AM, David Hildenbrand wrote: >>>>>>>>> It would still be a fairly coarse-grained locking, I am not sure if that >>>>>>>>> is a step into the right direction. If you want to modify *some* page >>>>>>>>> table in your process you have exclude each and every page table walker. >>>>>>>>> Or did I mis-interpret what you were saying? >>>>>>>> >>>>>>>> That is one possible design, it favours fast walking and penalizes >>>>>>>> mutation. We could also stick a lock in the PMD (instead of a >>>>>>>> refcount) and still logically be using a lock instead of a refcount >>>>>>>> scheme. Remember modify here is "want to change a table pointer into a >>>>>>>> leaf pointer" so it isn't an every day activity.. >>>>>>> >>>>>>> It will be if we somewhat frequent when reclaim an empty PTE page table >>>>>>> as soon as it turns empty. This not only happens when zapping, but also >>>>>>> during writeback/swapping. So while writing back / swapping you might be >>>>>>> left with empty page tables to reclaim. >>>>>>> >>>>>>> Of course, this is the current approach. Another approach that doesn't >>>>>>> require additional refcounts is scanning page tables for empty ones and >>>>>>> reclaiming them. This scanning can either be triggered manually from >>>>>>> user space or automatically from the kernel. >>>>>> >>>>>> Whether it is introducing a special rwsem or scanning an empty page >>>>>> table, there are two problems as follows: >>>>>> >>>>>> #1. When to trigger the scanning or releasing? >>>>> >>>>> For example when reclaiming memory, when scanning page tables in >>>>> khugepaged, or triggered by user space (note that this is the approach I >>>>> originally looked into). But it certainly requires more locking thought >>>>> to avoid stopping essentially any page table walker. >>>>> >>>>>> #2. Every time to release a 4K page table page, 512 page table >>>>>> entries need to be scanned. >>>>> >>>>> It would happen only when actually trigger reclaim of page tables >>>>> (again, someone has to trigger it), so it's barely an issue. >>>>> >>>>> For example, khugepaged already scans the page tables either way. >>>>> >>>>>> >>>>>> For #1, if the scanning is triggered manually from user space, the >>>>>> kernel is relatively passive, and the user does not fully know the best >>>>>> timing to scan. If the scanning is triggered automatically from the >>>>>> kernel, that is great. But the timing is not easy to confirm, is it >>>>>> scanned and reclaimed every time zap or try_to_unmap? >>>>>> >>>>>> For #2, refcount has advantages. >>>>>> >>>>>>> >>>>>>>> >>>>>>>> There is some advantage with this thinking because it harmonizes well >>>>>>>> with the other stuff that wants to convert tables into leafs, but has >>>>>>>> to deal with complicated locking. >>>>>>>> >>>>>>>> On the other hand, refcounts are a degenerate kind of rwsem and only >>>>>>>> help with freeing pages. It also puts more atomics in normal fast >>>>>>>> paths since we are refcounting each PTE, not read locking the PMD. >>>>>>>> >>>>>>>> Perhaps the ideal thing would be to stick a rwsem in the PMD. read >>>>>>>> means a table cannot be come a leaf. I don't know if there is space >>>>>>>> for another atomic in the PMD level, and we'd have to use a hitching >>>>>>>> post/hashed waitq scheme too since there surely isn't room for a waitq >>>>>>>> too.. >>>>>>>> >>>>>>>> I wouldn't be so quick to say one is better than the other, but at >>>>>>>> least let's have thought about a locking solution before merging >>>>>>>> refcounts :) >>>>>>> >>>>>>> Yes, absolutely. I can see the beauty in the current approach, because >>>>>>> it just reclaims "automatically" once possible -- page table empty and >>>>>>> nobody is walking it. The downside is that it doesn't always make sense >>>>>>> to reclaim an empty page table immediately once it turns empty. >>>>>>> >>>>>>> Also, it adds complexity for something that is only a problem in some >>>>>>> corner cases -- sparse memory mappings, especially relevant for some >>>>>>> memory allocators after freeing a lot of memory or running VMs with >>>>>>> memory ballooning after inflating the balloon. Some of these use cases >>>>>>> might be good with just triggering page table reclaim manually from user >>>>>>> space. >>>>>>> >>>>>> >>>>>> Yes, this is indeed a problem. Perhaps some flags can be introduced so >>>>>> that the release of page table pages can be delayed in some cases. >>>>>> Similar to the lazyfree mechanism in MADV_FREE? >>>>> >>>>> The issue AFAIU is that once your refcount hits 0 (no more references, >>>>> no more entries), the longer you wait with reclaim, the longer others >>>>> have to wait for populating a fresh page table because the "page table >>>>> to be reclaimed" is still stuck around. You'd have to keep the refcount >>>>> increased for a while, and only drop it after a while. But when? And >>>>> how? IMHO it's not trivial, but maybe there is an easy way to achieve it. >>>>> >>>> >>>> For running VMs with memory ballooning after inflating the balloon, is >>>> this a hot behavior? Even if it is, it is already facing the release and >>>> reallocation of physical pages. The overhead after introducing >>>> pte_refcount is that we need to release and re-allocate page table page. >>>> But 2MB physical pages only corresponds to 4KiB of PTE page table page. >>>> So maybe the overhead is not big. >>> >>> The cases that come to my mind are >>> >>> a) Swapping on shared memory with concurrent access >>> b) Reclaim on file-backed memory with concurrent access >>> c) Free page reporting as implemented by virtio-balloon >>> >>> In all of these cases, you can have someone immediately re-access the >>> page table and re-populate it. >> >> In the performance test shown on the cover, we repeatedly performed >> touch and madvise(MADV_DONTNEED) actions, which simulated the case >> you said above. >> >> We did find a small amount of performance regression, but I think it is >> acceptable, and no new perf hotspots have been added. > > That test always accesses 2MiB and does it from a single thread. Things > might (IMHO will) look different when only accessing individual pages > and doing the access from one/multiple separate threads (that's what
No, it includes multi-threading:
while (1) { char *c; char *start = mmap_area[cpu]; char *end = mmap_area[cpu] + FAULT_LENGTH; pthread_barrier_wait(&barrier); //printf("fault into %p-%p\n",start, end);
for (c = start; c < end; c += PAGE_SIZE) *c = 0;
pthread_barrier_wait(&barrier); for (i = 0; cpu==0 && i < num; i++) madvise(mmap_area[i], FAULT_LENGTH, MADV_DONTNEED); pthread_barrier_wait(&barrier); }
Thread on cpu0 will use madvise(MADV_DONTNEED) to release the physical memory of threads on other cpu.
> a),b) and c) essentially do, they don't do it in the pattern you > measured. what you measured matches rather a typical memory allocator). > >
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