Re: [External] Re: [PATCH v2 00/19] Free some vmemmap pages of hugetlb page

[Muchun Song]

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). So I think that the overhead is acceptable.

Thanks.

>
> Thanks!
> --
> Michal Hocko
> SUSE Labs



-- 
Yours,
Muchun