Hi All, Firstly, thank you for your careful review and attention to my patch (and apologies for top-posting!). Let me first explain why our use case requires hugetlb over THP and then elaborate on the difficulty we have to maintain the correct number of hugepages in the pool, finally concluding with why the proposed approach would help us. Hopefully you can extend it to other use cases and justify the proposal. We use Linux to operate a KVM-based hypervisor. Using hugepages to back VM memory significantly increases performance and density. Each VM incurs a 4k regular page overhead which can vary drastically even at runtime (eg. depending on network traffic). In addition, the software doesn't know upfront if users will power on one large VM or several small VMs. To manage the varying balance of 4k pages vs. hugepages, we originally leveraged THP. However, constant fragmentation due to VM power cycles, the varying overhead I mentioned above, and other operations like reconfiguration of NIC RX buffers resulted in two problems: 1) There were no guarantees hugepages would be used; and 2) Constant memory compaction incurred a measurable overhead. Having a userspace service managing hugetlb gave us significant performance advantages and much needed determinism. It chooses when to try and create more hugepages as well as how many hugepages to go after. Elements like how many hugepages it actually gets, combined with what operations are happening on the host, allow our service to make educated decisions about when to compact memory, drop caches, and retry growing (or shrinking) the pool. But that comes with a challenge: despite listening on cgroup for pressure notifications (which happen from those runtime events we do not control), the service is not guaranteed to sacrifice hugepages fast enough and that causes an OOM. The killer will normally take out a VM even if there are plenty of unused hugepages and that's obviously disruptive for users. For us, free hugepages are almost always expendable. For the bloat cases which are predictable, a memory management service can adjust the hugepage pool size ahead of time. But it can be hard to anticipate all scenarios, and some can be very volatile. Having a failsafe mechanism as proposed in this patch offers invaluable protection when things are missed. The proposal solves this problem by sacrificing hugepages inline even when the pressure comes from kernel allocations. The userspace service can later readjust the pool size without being under pressure. Given this is configurable, and defaults to being off, we thought it would be a nice addition to the kernel and appreciated by other users that may have similar requirements. I welcome your comments and thank you again for your time! Eiichi > On Feb 17, 2021, at 16:57, Michal Hocko <mhocko@xxxxxxxx> wrote: > > On Tue 16-02-21 14:30:15, Mike Kravetz wrote: > [...] >> However, this is an 'opt in' feature. So, I would not expect anyone who >> carefully plans the size of their hugetlb pool to enable such a feature. >> If there is a use case where hugetlb pages are used in a non-essential >> application, this might be of use. > > I would really like to hear about the specific usecase. Because it > smells more like a misconfiguration. What would be non-essential hugetlb > pages? This is not a resource to be pre-allocated just in case, right? > > -- > Michal Hocko > SUSE Labs
