Re: committed memory, mmaps and shms
From: Marcos Dione
Date: Thu Mar 12 2015 - 12:56:09 EST
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.
> > 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
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.
If someone is really interested in that, it would have to poll an
interesting amount of files, but definitely doable. Just cat'ing one of
these files for a process with 128 mmaps and 1 shm as before gave these
> > I understand what /proc/sys/vm/overcommit_memory is for; what I
> > don't understand is what exactly counted in the Committed_AS line in
> > /proc/meminfo.
> It accounts all the address space reservations - e.g. mmap(len), len
> will get added. The things are slightly more complicated but start
> looking at callers of security_vm_enough_memory_mm should give you an
> idea what everything is included.
> How is this number used depends on the overcommit mode.
> __vm_enough_memory would give you a better picture.
> > I also read Documentation/vm/overcommit-accounting
> What would help you to understand it better?
I think that after this dip in terminology I should go back to it
and try again to figure it out myself :) Of course findings will be
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