On Wed 14-08-19 21:51:47, Yang Shi wrote: > > > On 8/14/19 4:08 AM, Michal Hocko wrote: > > On Mon 12-08-19 10:00:17, Yang Shi wrote: > > > > > > On 8/12/19 2:34 AM, Michal Hocko wrote: > > > > On Fri 09-08-19 16:54:43, Yang Shi wrote: > > > > > On 8/9/19 11:26 AM, Yang Shi wrote: > > > > > > On 8/9/19 11:02 AM, Michal Hocko wrote: > > > > [...] > > > > > > > I have to study the code some more but is there any reason why those > > > > > > > pages are not accounted as proper THPs anymore? Sure they are partially > > > > > > > unmaped but they are still THPs so why cannot we keep them accounted > > > > > > > like that. Having a new counter to reflect that sounds like papering > > > > > > > over the problem to me. But as I've said I might be missing something > > > > > > > important here. > > > > > > I think we could keep those pages accounted for NR_ANON_THPS since they > > > > > > are still THP although they are unmapped as you mentioned if we just > > > > > > want to fix the improper accounting. > > > > > By double checking what NR_ANON_THPS really means, > > > > > Documentation/filesystems/proc.txt says "Non-file backed huge pages mapped > > > > > into userspace page tables". Then it makes some sense to dec NR_ANON_THPS > > > > > when removing rmap even though they are still THPs. > > > > > > > > > > I don't think we would like to change the definition, if so a new counter > > > > > may make more sense. > > > > Yes, changing NR_ANON_THPS semantic sounds like a bad idea. Let > > > > me try whether I understand the problem. So we have some THP in > > > > limbo waiting for them to be split and unmapped parts to be freed, > > > > right? I can see that page_remove_anon_compound_rmap does correctly > > > > decrement NR_ANON_MAPPED for sub pages that are no longer mapped by > > > > anybody. LRU pages seem to be accounted properly as well. As you've > > > > said NR_ANON_THPS reflects the number of THPs mapped and that should be > > > > reflecting the reality already IIUC. > > > > > > > > So the only problem seems to be that deferred THP might aggregate a lot > > > > of immediately freeable memory (if none of the subpages are mapped) and > > > > that can confuse MemAvailable because it doesn't know about the fact. > > > > Has an skewed counter resulted in a user observable behavior/failures? > > > No. But the skewed counter may make big difference for a big scale cluster. > > > The MemAvailable is an important factor for cluster scheduler to determine > > > the capacity. > > But MemAvailable is a very rough estimation. Is relying on it really a > > good measure? I mean there is a lot of reclaimable memory that is not > > reflected there (some fs. internal data structures, networking buffers > > etc.) > > Yes, I agree there are other freeable objects not accounted into > MemAvailable. Their size depends on the workload. But, deferred split THPs > seems more common with the common workloads. A simple run with MariaDB test > of mmtest shows it could generate over fifteen thousand deferred split THPs > (accumulated around 30G in one hour run, 75% of 40G memory for my VM). So, > it may be worth accounting deferred split THPs in MemAvailable. This is a very useful information to put into the changelog. -- Michal Hocko SUSE Labs
