Accounting for "missing RAM"

[Arun Sharma <asharma@fb.com>]

I wrote a script to parse /proc/zoneinfo to figure out how memory on my 
server was getting used.

On many machines this is able to account for RAM +/- 10MB (which I 
consider within the margin of error).

But on many machines, there is 2-3GB of RAM that's unaccounted for and 
mysteriously missing from zoneinfo as well as /proc/meminfo.

When I parse /proc/kpageflags, pages with flags==0 correspond to 
nr_free_pages and pages with flags=KPF_BUDDY correspond to the number of 
"missing ram pages".

As far as I understand, KPF_BUDDY is set only on the first page of a 
higher order page. So if for some reason, we were counting an order=3 
page as 7 pages instead of 8, it'd explain what I am seeing.

  -Arun

#!/usr/bin/env python

import sys, re, string, os

class NestedDict(dict):
     """Implementation of perl's autovivification feature."""
     def __getitem__(self, item):
         try:
             return dict.__getitem__(self, item)
         except KeyError:
             value = self[item] = type(self)()
             return value

sep = re.compile('^Node (.*)')
pages = re.compile('.*nr.*pages.*')
slab = re.compile('.*nr.*slab.*')
present = re.compile('.*present.*')
pcpu_pageset_count = re.compile('.*count:.*')
zone = None
page_size = os.sysconf('SC_PAGESIZE')

zones = NestedDict()
for line in open('/proc/zoneinfo').readlines():
     m = sep.match(line)
     if m:
         zone = m.group(1)
         zones[zone]['pcpu_pageset'] = 0
         # Old kernels don't have this in /proc/zoneinfo
         zones[zone]['nr_free_pages'] = 0
     m1 = pages.match(line)
     m2 = slab.match(line)
     m3 = present.match(line)
     if m1 or m2 or m3:
         name, val = string.split(line)
         val = int(val)
         zones[zone][name] = val
         continue
     m4 = pcpu_pageset_count.match(line)
     if m4:
         name, val = string.split(line)
         val = int(val)
         zones[zone]['pcpu_pageset'] += val

global_diff = 0L
for z in zones.keys():
     allocated = zones[z]['present'] - zones[z]['nr_free_pages']
     if allocated < 0: continue
     print '##############'
     print z
     accounted = 0
     for k in ('nr_file_pages', 'nr_anon_pages',
               'nr_slab_reclaimable', 'nr_slab_unreclaimable',
               'nr_page_table_pages', 'pcpu_pageset'):
         val = zones[z][k]
         print k, val
         accounted += val
     print "allocated", "accounted", "diff"
     print allocated, accounted, allocated - accounted
     global_diff += (allocated - accounted)

vmalloc = 0L
vmalloc_re = re.compile('.*VmallocUsed:\s+(\d+).*')
for line in open('/proc/meminfo').readlines():
     m = vmalloc_re.match(line)
     if m:
         vmalloc_kb = int(m.group(1))
vmalloc = vmalloc_kb * 1024/page_size

print '##############'
print 'vmalloc',  vmalloc
print "missing ram: ", (global_diff - vmalloc) * page_size

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