On 20.11.21 21:12, Shakeel Butt wrote: > Many applications do sophisticated management of their heap memory for > better performance but with low cost. We have a bunch of such > applications running on our production and examples include caching and > data storage services. These applications keep their hot data on the > THPs for better performance and release the cold data through > MADV_DONTNEED to keep the memory cost low. > > The kernel defers the split and release of THPs until there is memory > pressure. This causes complicates the memory management of these > sophisticated applications which then needs to look into low level > kernel handling of THPs to better gauge their headroom for expansion. Can you elaborate a bit on that point? What exactly does such an application do? I would have assumed that it's mostly transparent for applications. > In > addition these applications are very latency sensitive and would prefer > to not face memory reclaim due to non-deterministic nature of reclaim. That makes sense. > > This patch let such applications not worry about the low level handling > of THPs in the kernel and splits the THPs synchronously on > MADV_DONTNEED. The main user I'm concerned about is virtio-balloon, which ends up discarding VM memory via MADV_DONTNEED when inflating the balloon in the guest in 4k granularity, but also during "free page reporting" continuously when e.g., a 2MiB page becomes free in the guest. We want both activities to be fast, and especially during "free page reporting", to defer any heavy work. Do we have a performance evaluation how much overhead is added e.g., for a single 4k MADV_DONTNEED call on a THP or on a MADV_DONTNEED call that covers the whole THP? -- Thanks, David / dhildenb
