On Sat, Feb 6, 2021 at 12:01 AM Joao Martins <joao.m.martins@xxxxxxxxxx> wrote: > > On 2/4/21 3:50 AM, Muchun Song wrote: > > Hi all, > > > > [...] > > > When a HugeTLB is freed to the buddy system, we should allocate 6 pages for > > vmemmap pages and restore the previous mapping relationship. > > > > Apart from 2MB HugeTLB page, we also have 1GB HugeTLB page. It is similar > > to the 2MB HugeTLB page. We also can use this approach to free the vmemmap > > pages. > > > > In this case, for the 1GB HugeTLB page, we can save 4094 pages. This is a > > very substantial gain. On our server, run some SPDK/QEMU applications which > > will use 1024GB hugetlbpage. With this feature enabled, we can save ~16GB > > (1G hugepage)/~12GB (2MB hugepage) memory. > > > > Because there are vmemmap page tables reconstruction on the freeing/allocating > > path, it increases some overhead. Here are some overhead analysis. > > [...] > > > Although the overhead has increased, the overhead is not significant. Like Mike > > said, "However, remember that the majority of use cases create hugetlb pages at > > or shortly after boot time and add them to the pool. So, additional overhead is > > at pool creation time. There is no change to 'normal run time' operations of > > getting a page from or returning a page to the pool (think page fault/unmap)". > > > > Despite the overhead and in addition to the memory gains from this series ... > there's an additional benefit there isn't talked here with your vmemmap page > reuse trick. That is page (un)pinners will see an improvement and I presume because > there are fewer memmap pages and thus the tail/head pages are staying in cache more > often. > > Out of the box I saw (when comparing linux-next against linux-next + this series) > with gup_test and pinning a 16G hugetlb file (with 1G pages): > > get_user_pages(): ~32k -> ~9k > unpin_user_pages(): ~75k -> ~70k > > Usually any tight loop fetching compound_head(), or reading tail pages data (e.g. > compound_head) benefit a lot. There's some unpinning inefficiencies I am fixing[0], but > with that in added it shows even more: > > unpin_user_pages(): ~27k -> ~3.8k > > FWIW, I was also seeing that with devdax and the ZONE_DEVICE vmemmap page reuse equivalent > series[1] but it was mixed with other numbers. It's really a surprise. Thank you very much for the test data. Very nice. Thanks again. > > Anyways, JFYI :) > > Joao > > [0] https://lore.kernel.org/linux-mm/20210204202500.26474-1-joao.m.martins@xxxxxxxxxx/ > [1] https://lore.kernel.org/linux-mm/20201208172901.17384-1-joao.m.martins@xxxxxxxxxx/
