On Mon, Feb 18, 2019 at 03:47:22PM -0800, Alexander Duyck wrote: > > > So far with my patch set that hints at the PMD level w/ THP enabled I > > > am not really seeing that much overhead for the hypercalls. The bigger > > > piece that is eating up CPU time is all the page faults and page > > > zeroing that is going on as we are cycling the memory in and out of > > > the guest. Some of that could probably be resolved by using MADV_FREE, > > > but if we are under actual memory pressure I suspect it would behave > > > similar to MADV_DONTNEED. > > > > > > > MADV_FREE is certainly the better thing to do for hinting in my opinion. > > It should result in even less overhead. Thanks for the comment about the > > hypercall overhead. > > Yeah, no problem. The only thing I don't like about MADV_FREE is that > you have to have memory pressure before the pages really start getting > scrubbed with is both a benefit and a drawback. Basically it defers > the freeing until you are under actual memory pressure so when you hit > that case things start feeling much slower, that and it limits your > allocations since the kernel doesn't recognize the pages as free until > it would have to start trying to push memory to swap. For sure if someone *wants* to spend cycles freeing memory, we could add a system call that does exactly that. There's no reason to force that on the same CPU while VCPU is stopped though. -- MST
