top shows only half of realmem?

[John Baldwin]

On Wednesday, January 26, 2011 1:04:02 pm Marco van Tol wrote:
> On Wed, Jan 26, 2011 at 12:35:56PM -0500, John Baldwin wrote:
> > On Wednesday, January 26, 2011 8:20:28 am Bartosz Stec wrote:
> > > W dniu 2011-01-26 14:06, John Baldwin pisze:
> > > > On Wednesday, January 26, 2011 7:20:34 am Bartosz Stec wrote:
> > > >> Guys,
> > > >>
> > > >> could someone explain me this?
> > > >>
> > > >>      # sysctl hw.realmem
> > > >>      hw.realmem: 2139029504
> > > >>
> > > >> top line shows:
> > > >>
> > > >>      Mem: 32M Active, 35M Inact, 899M Wired, 8392K Cache, 199M Buf, 58M Free
> > > >>
> > > >> 32+35+899+8+199+58 = 1231MB
> > > >>
> > > >> Shouldn't that sum to all available ram? Or maybe I'm reading it wrong?
> > > >> This machine has indeed 2GB of ram on board and showed in BIOS.
> > > >> i386  FreeBSD 8.2-PRERELEASE #16: Mon Jan 17 22:28:53 CET 2011
> > > >> Cheers.
> > > > First, don't include 'buf' as isn't a separate set of RAM, it is only a range
> > > > of the virtual address space in the kernel.  It used to be relevant when the
> > > > buffer cache was separate from the VM page cache, but now it is mostly
> > > > irrelevant (arguably it should just be dropped from top output).
> > > 
> > > Thanks for the explanation. So 1231MB - 199MB Buf and we got about 1GB 
> > > of memory instead of 2B.
> > > 
> > > > However, look at what hw.physmem says (and the realmem and availmem lines in
> > > > dmesg).  realmem is actually not that useful as it is not a count of the
> > > > amount of memory, but the address of the highest memory page available.  There
> > > > can be less memory available than that due to "holes" in the address space for
> > > > PCI memory BARs, etc.
> > > >
> > > OK, here you go:
> > > # sysctl hw | grep mem
> > > 
> > >     hw.physmem: 2125893632
> > >     hw.usermem: 1212100608
> > >     hw.realmem: 2139029504
> > >     hw.pci.host_mem_start: 2147483648
> > 
> > Humm, you should still have 2GB of RAM then.  All the memory you set aside
> > for ARC should be counted in the 'wired' count, so I'm not sure why you see
> > 1GB of RAM rather than 2GB.
> 
> For what its worth (seems to be the same values top shows), the sysctl's
> I use to make cacti graphs of memory usage are: (Counts are in pages)
> 
> vm.stats.vm.v_page_size
> 
> vm.stats.vm.v_wire_count
> vm.stats.vm.v_active_count
> vm.stats.vm.v_inactive_count
> vm.stats.vm.v_cache_count
> vm.stats.vm.v_free_count
> 
> Using the output of those sysctls I allways get a cacti graph which at
> least very much seems to account for all memory, and has a flat surface
> in a stacked graph.

These sysctls are exactly what top uses.  There is also a 'v_page_count'
which is a total count of pages.

-- 
John Baldwin