What is the concept behind “Lost RAM” which appears in Dumpsys meminfo?

Question

As I have mentioned in question, Lost RAM appears in Dumpsys meminfo.

1. What is the concept behind “Lost RAM” which appears in Dumpsys meminfo?
2. What is its significance in Kitkat. How it can be reclaimed and used?

Sample dumpsys showing "Lost RAM".

Total RAM: 998096 kB
 Free RAM: 574945 kB (145869 cached pss + 393200 cached + 35876 free)

 Used RAM: 392334 kB (240642 used pss + 107196 buffers + 3856 shmem + 40640 slab)

 Lost RAM: 30817 kB

   Tuning: 64 (large 384), oom 122880 kB, restore limit 40960 kB (high-end-gfx)

Answer 1

On my system, they are caused mostly by ION (which replaces pmem). If ion debug is enabled with your kernel, you can calculate your ION usage with this script:

adb shell cat /d/ion/heaps/system|perl -ne 'chomp; if (m/pages in.*pool = (\d+) total/) {$x += $1;} if (m/^\s+total\s+(\d+)$/) {$y += $1} END {printf "use: %d kb, cache: %d kb; total: %d kb", $y/1024, $x/1024, ($x + $y)/1024}'

In fact, any kernel page allocation done and tracked by drivers will not be tracked by the kernel, thus counting for the lost ram.

After I stop the system server, I used another .awk script to calculate lost ram (dumpsys meminfo will require the meminfo service and thus won't work anymore), and the lost ram is very closely following ION debug output:

#!/usr/bin/awk

BEGIN {
    types["MemTotal"] = 1;
    types["Pss"] = 1;
    types["MemFree"] = 1;
    types["Cached"] = 1;
    types["Buffers"] = 1;
    types["Shmem"] = 1;
    types["Slab"] = 1;
}


## start code-generator "^\\s *#"
#echo
# for x in Pss MemTotal MemFree Cached Buffers Shmem Slab; do
#     cat << EOF
#/$x: / {
#     hash["$x"] += \$2;
# next
#}
#
#EOF
# done
## end code-generator
## start generated code

/Pss: / {
    hash["Pss"] += $2;
    next
 }

 /MemTotal: / {
     hash["MemTotal"] += $2;
     next
 }

 /MemFree: / {
     hash["MemFree"] += $2;
     next
 }

 /Cached: / {
     hash["Cached"] += $2;
     next
 }

 /Buffers: / {
     hash["Buffers"] += $2;
     next
 }

 /Shmem: / {
     hash["Shmem"] += $2;
     next
 }

 /Slab: / {
     hash["Slab"] += $2;
     next
 }


## end generated code

END {
    lost =  0;
    for (type in types) {
        if (type == "MemTotal") {
            lost += hash[type];
        } else {
            lost -= hash[type];
        }
    }
    print "lost: " lost " kB\n";
}

I also checked again after I force a kernel memory shrink with adb shell sh -c 'echo 3 > /proc/sys/vm/drop_caches', the results are still very close.