On 21.12.20 17:25, Liang Li wrote: > The first version can be found at: https://lkml.org/lkml/2020/4/12/42 > > Zero out the page content usually happens when allocating pages with > the flag of __GFP_ZERO, this is a time consuming operation, it makes > the population of a large vma area very slowly. This patch introduce > a new feature for zero out pages before page allocation, it can help > to speed up page allocation with __GFP_ZERO. > > My original intention for adding this feature is to shorten VM > creation time when SR-IOV devicde is attached, it works good and the > VM creation time is reduced by about 90%. > > Creating a VM [64G RAM, 32 CPUs] with GPU passthrough > ===================================================== > QEMU use 4K pages, THP is off > round1 round2 round3 > w/o this patch: 23.5s 24.7s 24.6s > w/ this patch: 10.2s 10.3s 11.2s > > QEMU use 4K pages, THP is on > round1 round2 round3 > w/o this patch: 17.9s 14.8s 14.9s > w/ this patch: 1.9s 1.8s 1.9s > ===================================================== > I am still not convinces that we want/need this for this (main) use case. Why can't we use huge pages for such use cases (that really care about VM creation time) and rather deal with pre-zeroing of huge pages instead? If possible, I'd like to avoid GFP_ZERO (for reasons already discussed). > Obviously, it can do more than this. We can benefit from this feature > in the flowing case: > > Interactive sence > ================= > Shorten application lunch time on desktop or mobile phone, it can help > to improve the user experience. Test shows on a > server [Intel(R) Xeon(R) CPU E5-2620 v3 @ 2.40GHz], zero out 1GB RAM by > the kernel will take about 200ms, while some mainly used application > like Firefox browser, Office will consume 100 ~ 300 MB RAM just after > launch, by pre zero out free pages, it means the application launch > time will be reduced about 20~60ms (can be visual sensed?). May be > we can make use of this feature to speed up the launch of Andorid APP > (I didn't do any test for Android). I am not really sure if you can actually visually sense a difference in your examples. Startup time of an application is not just memory allocation (page zeroing) time. It would be interesting of much of a difference this actually makes in practice. (e.g., firefox startup time etc.) > > Virtulization > ============= > Speed up VM creation and shorten guest boot time, especially for PCI > SR-IOV device passthrough scenario. Compared with some of the para > vitalization solutions, it is easy to deploy because it’s transparent > to guest and can handle DMA properly in BIOS stage, while the para > virtualization solution can’t handle it well. What is the "para virtualization" approach you are talking about? > > Improve guest performance when use VIRTIO_BALLOON_F_REPORTING for memory > overcommit. The VIRTIO_BALLOON_F_REPORTING feature will report guest page > to the VMM, VMM will unmap the corresponding host page for reclaim, > when guest allocate a page just reclaimed, host will allocate a new page > and zero it out for guest, in this case pre zero out free page will help > to speed up the proccess of fault in and reduce the performance impaction. Such faults in the VMM are no different to other faults, when first accessing a page to be populated. Again, I wonder how much of a difference it actually makes. > > Speed up kernel routine > ======================= > This can’t be guaranteed because we don’t pre zero out all the free pages, > but is true for most case. It can help to speed up some important system > call just like fork, which will allocate zero pages for building page > table. And speed up the process of page fault, especially for huge page > fault. The POC of Hugetlb free page pre zero out has been done. Would be interesting to have an actual example with some numbers. > > Security > ======== > This is a weak version of "introduce init_on_alloc=1 and init_on_free=1 > boot options", which zero out page in a asynchronous way. For users can't > tolerate the impaction of 'init_on_alloc=1' or 'init_on_free=1' brings, > this feauture provide another choice. "we don’t pre zero out all the free pages" so this is of little actual use. -- Thanks, David / dhildenb _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
