On Tue 20-04-21 14:24:30, Matthew Wilcox wrote: > On Tue, Apr 20, 2021 at 03:13:54PM +0300, Mike Rapoport wrote: > > Add a paragraph that explains that it may happen that the counters in > > /proc/meminfo do not add up to the overall memory usage. > > ... that is, the sum may be lower because memory is allocated for other > purposes that is not reported here, right? yes. Many direct page allocator users are not accounted in any of the existing counters. > Is it ever possible for it to be higher? Maybe due to a race when > sampling the counters? Yes likely possible. You will never get an atomic snapshot of all counters. > > Provides information about distribution and utilization of memory. This > > -varies by architecture and compile options. The following is from a > > -16GB PIII, which has highmem enabled. You may not have all of these fields. > > +varies by architecture and compile options. Please note that it may happen > > +that the memory accounted here does not add up to the overall memory usage > > +and the difference for some workloads can be substantial. In many cases there > > +are other means to find out additional memory using subsystem specific > > +interfaces, for instance /proc/net/sockstat for TCP memory allocations. > > How about just: > > +varies by architecture and compile options. The memory reported here > +may not add up to the overall memory usage and the difference for some > +workloads can be substantial. [...] > > But I'd like to be a bit more explicit about the reason, hence my question > above to be sure I understand. > > > It's also not entirely clear which of the fields in meminfo can be > usefully summed. VmallocTotal is larger than MemTotal, for example. Yes. Many/Most counters cannot be simply sumed up. A trivial example would be Active/Inactive is a sum of both anona and file. Mlocked will be accounted in LRU pages and Unevictable. MemAvailable is not really a counter... Usual memory consumption is usually something like LRU pages + Slab memory + kernel stack + vmalloc used + pcp. > But I know that KernelStack is allocated through vmalloc these days, > and I don't know whether VmallocUsed includes KernelStack or whether I > can sum them. Similarly, is Mlocked a subset of Unevictable? > > There is some attempt at explaining how these numbers fit together, but > it's outdated, and doesn't include Mlocked, Unevictable or KernelStack Agreed there is a lot of tribal knowledge or even misconceptions flying around and it will take much more work to put everything into shape. This is only one tiny step forward. -- Michal Hocko SUSE Labs
