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SubjectRe: committed memory, mmaps and shms
On Thu 12-03-15 13:56:00, Marcos Dione wrote:
> On Thu, Mar 12, 2015 at 11:35:13AM -0400, Michal Hocko wrote:
> > > > On Wed 11-03-15 19:10:44, Marcos Dione wrote:
> > [...]
> > > > > $ free
> > > > > total used free shared buffers cached
> > > > > Mem: 396895176 395956332 938844 0 8972 356409952
> > > > > -/+ buffers/cache: 39537408 357357768
> > > > > Swap: 8385788 8385788 0
> > > > >
> > > > > This reports 378GiB of RAM, 377 used; of those 8MiB in buffers,
> > > > > 339GiB in cache, leaving only 38Gib for processes (for some reason this
> > > >
> > > > I am not sure I understand your math here. 339G in the cache should be
> > > > reclaimable (be careful about the shmem though). It is the rest which
> > > > might be harder to reclaim.
> > >
> > > These 38GiB I mention is the rest of 378 available minus 339 in
> > > cache. To me this difference represents the sum of the resident
> > > anonymous memory malloc'ed by all processes. Unless there's some othr
> > > kind of pages accounted in 'Used'.
> >
> > The kernel needs memory as well for its internal data structures
> > (stacks, page tables, slab objects, memory used by drivers and what not).
>
> Are those in or out of the total memory reported by free? I had the
> impression the were out. 396895176 accounts only for 378.5GiB of the 384
> available in the machine; I assumed the missing 5.5 was kernel memory.

I haven't checked the code of `free' but I would expect this to be part
of `used'.

> > > Yes, but my question was more on the lines of 'why free or
> > > /proc/meminfo do not show it'. Maybe it's just that it's difficult to
> > > define (like I said, "sum of resident anonymous..." &c) or nobody really
> > > cares about this. Maybe I shouldn't either.
> >
> > meminfo is exporting this information as AnonPages.
>
> I think that what I'm trying to do is figure out what each value
> represents and where it's incuded, as if to make a graph like this
> (fields in /proc/meminfo between []'s; dots are inactive, plus signs
> active):
>
> RAM swap other (mmaps)
> |------------------------------|-----------------------------|-------------...
> |.| kernel [Slab+KernelStack+PageTables+?]
> |.| buffers [Buffers]
> | . . . . .. .| swap cached (not necesarily like this, but you get the idea) (I'm assuming that it only includes anon pages, shms and private mmaps) [SwapCached]
> |++..| resident annon (malloc'ed) [AnonPages/Active/Inactive(anon)]
> |+++....+++........| cache [Cached/Active/Inactive(file)]
> |+++...| (resident?) shms [Shmem]
> |+++..| resident mmaps
> |.....| other fs cache
> |..| free [MemFree]
> |.............| used swap [SwapTotal-SwapFree]
> |...............| swap free [SwapFree]
>
> Note that there are no details on how the swap is used between anon
> pages, shm and others; neither about mmaps; except in /proc/<pid>/smaps.

Well, the memory management subsystem is rather complex and it is not
really trivial to match all the possible combinations into simple
counters.

I would be interested in the particular usecase where you want the
specific information and it is important outside of debugging purposes.

--
Michal Hocko
SUSE Labs


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