On Fri 30-10-20 18:24:25, Muchun Song wrote: > On Fri, Oct 30, 2020 at 5:14 PM Michal Hocko <mhocko@xxxxxxxx> wrote: > > > > On Mon 26-10-20 22:50:55, Muchun Song wrote: > > > If we uses the 1G hugetlbpage, we can save 4095 pages. This is a very > > > substantial gain. On our server, run some SPDK/QEMU applications which > > > will use 1000GB hugetlbpage. With this feature enabled, we can save > > > ~16GB(1G hugepage)/~11GB(2MB hugepage) memory. > > [...] > > > 15 files changed, 1091 insertions(+), 165 deletions(-) > > > create mode 100644 include/linux/bootmem_info.h > > > create mode 100644 mm/bootmem_info.c > > > > This is a neat idea but the code footprint is really non trivial. To a > > very tricky code which hugetlb is unfortunately. > > > > Saving 1,6% of memory is definitely interesting especially for 1GB pages > > which tend to be more static and where the savings are more visible. > > > > Anyway, I haven't seen any runtime overhead analysis here. What is the > > price to modify the vmemmap page tables and make them pte rather than > > pmd based (especially for 2MB hugetlb). Also, how expensive is the > > vmemmap page tables reconstruction on the freeing path? > > Yeah, I haven't tested the remapping overhead of reserving a hugetlb > page. I can do that. But the overhead is not on the allocation/freeing of > each hugetlb page, it is only once when we reserve some hugetlb pages > through /proc/sys/vm/nr_hugepages. Once the reservation is successful, > the subsequent allocation, freeing and using are the same as before > (not patched). Yes, that is quite clear. Except for the hugetlb overcommit and migration if the pool is depeleted. Maybe few other cases. > So I think that the overhead is acceptable. Having some numbers for a such a large feature is really needed. -- Michal Hocko SUSE Labs
