On Mon, Mar 09, 2020 at 05:25:24PM -0700, Roman Gushchin wrote:
> Commit 944d9fec8d7a ("hugetlb: add support for gigantic page allocation
> at runtime") has added the run-time allocation of gigantic pages. However
> it actually works only at early stages of the system loading, when
> the majority of memory is free. After some time the memory gets
> fragmented by non-movable pages, so the chances to find a contiguous
> 1 GB block are getting close to zero. Even dropping caches manually
> doesn't help a lot.
>
> At large scale rebooting servers in order to allocate gigantic hugepages
> is quite expensive and complex. At the same time keeping some constant
> percentage of memory in reserved hugepages even if the workload isn't
> using it is a big waste: not all workloads can benefit from using 1 GB
> pages.
>
> The following solution can solve the problem:
> 1) On boot time a dedicated cma area* is reserved. The size is passed
> as a kernel argument.
> 2) Run-time allocations of gigantic hugepages are performed using the
> cma allocator and the dedicated cma area
>
> In this case gigantic hugepages can be allocated successfully with a
> high probability, however the memory isn't completely wasted if nobody
> is using 1GB hugepages: it can be used for pagecache, anon memory,
> THPs, etc.
>
> * On a multi-node machine a per-node cma area is allocated on each node.
> Following gigantic hugetlb allocation are using the first available
> numa node if the mask isn't specified by a user.
>
> Usage:
> 1) configure the kernel to allocate a cma area for hugetlb allocations:
> pass hugetlb_cma=10G as a kernel argument
>
> 2) allocate hugetlb pages as usual, e.g.
> echo 10 > /sys/kernel/mm/hugepages/hugepages-1048576kB/nr_hugepages
Btw, if somebody wants to give it a try, please pull these two following
fs-related fixes:
1) for btrfs: https://patchwork.kernel.org/patch/11420997/
2) for ext4: https://lore.kernel.org/patchwork/patch/1202868/
Please, make sure that you pull the ext4 fix, if, say, /boot is served
by ext4.
Thanks!
