On Wed 11-07-18 13:55:15, Wang, Wei W wrote: > On Wednesday, July 11, 2018 7:10 PM, Michal Hocko wrote: > > On Wed 11-07-18 18:52:45, Wei Wang wrote: > > > On 07/11/2018 05:21 PM, Michal Hocko wrote: > > > > On Tue 10-07-18 18:44:34, Linus Torvalds wrote: > > > > [...] > > > > > That was what I tried to encourage with actually removing the > > > > > pages form the page list. That would be an _incremental_ > > > > > interface. You can remove MAX_ORDER-1 pages one by one (or a > > > > > hundred at a time), and mark them free for ballooning that way. > > > > > And if you still feel you have tons of free memory, just continue > > removing more pages from the free list. > > > > We already have an interface for that. alloc_pages(GFP_NOWAIT, > > MAX_ORDER -1). > > > > So why do we need any array based interface? > > > > > > Yes, I'm trying to get free pages directly via alloc_pages, so there > > > will be no new mm APIs. > > > > OK. The above was just a rough example. In fact you would need a more > > complex gfp mask. I assume you only want to balloon only memory directly > > usable by the kernel so it will be > > (GFP_KERNEL | __GFP_NOWARN) & ~__GFP_RECLAIM > > Sounds good to me, thanks. > > > > > > I plan to let free page allocation stop when the remaining system free > > > memory becomes close to min_free_kbytes (prevent swapping). > > > > ~__GFP_RECLAIM will make sure you are allocate as long as there is any > > memory without reclaim. It will not even poke the kswapd to do the > > background work. So I do not think you would need much more than that. > > "close to min_free_kbytes" - I meant when doing the allocations, we > intentionally reserve some small amount of memory, e.g. 2 free page > blocks of "MAX_ORDER - 1". So when other applications happen to do > some allocation, they may easily get some from the reserved memory > left on the free list. Without that reserved memory, other allocation > may cause the system free memory below the WMARK[MIN], and kswapd > would start to do swapping. This is actually just a small optimization > to reduce the probability of causing swapping (nice to have, but not > mandatary because we will allocate free page blocks one by one). I really have hard time to follow you here. Nothing outside of the core MM proper should play with watermarks. > > But let me note that I am not really convinced how this (or previous) > > approach will really work in most workloads. We tend to cache heavily so > > there is rarely any memory free. > > With less free memory, the improvement becomes less, but should be > nicer than no optimization. For example, the Linux build workload > would cause 4~5 GB (out of 8GB) memory to be used as page cache at the > final stage, there is still ~44% live migration time reduction. But most systems will stay somewhere around the high watermark if there is any page cache activity. Especially after a longer uptime. -- Michal Hocko SUSE Labs
