Re: [PATCH v2] docs: proc.rst: meminfo: briefly describe gaps in memory accounting

On Tue 20-04-21 14:24:30, Matthew Wilcox wrote:

On Tue, Apr 20, 2021 at 03:13:54PM +0300, Mike Rapoport wrote:

quoted

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.

quoted

 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it's outdated, and doesn't include Mlocked, Unevictable or KernelStack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