title: Can not clear cache memory on Linux
body: <p>I have Logstash and Elasticsearch running on an EC2 (8GB ram), ArchLinux.</p>
  
  <p>I can not clear the cached memory (I understand kernel should be able to claim this memory back if needed). from the <code>meminfo</code>, I understand that the most of the cached memory is inactive, which means no process is using that,so I should be able to clear it, right?. </p>
  
  <p>I am doing <code>sync &amp;&amp; echo 3 &gt; /proc/sys/vm/drop_caches</code> to clear the memory.
  Only system reboot is clearing this memory.
  There could be a memory leak, how can I see which process is using this cache memory. </p>
  
  <pre><code>cat /proc/meminfo 
  MemTotal:        7661268 kB
  MemFree:          613144 kB
  MemAvailable:     561144 kB
  Buffers:            8580 kB
  Cached:          2271144 kB
  SwapCached:            0 kB
  Active:           165424 kB
  Inactive:        2244356 kB
  Active(anon):     152528 kB
  Inactive(anon):  2226088 kB
  Active(file):      12896 kB
  Inactive(file):    18268 kB
  Unevictable:     4429248 kB
  Mlocked:         4429248 kB
  SwapTotal:             0 kB
  SwapFree:              0 kB
  Dirty:               164 kB
  Writeback:             0 kB
  AnonPages:       4559304 kB
  Mapped:            33916 kB
  Shmem:           2226220 kB
  Slab:              54908 kB
  SReclaimable:      33764 kB
  SUnreclaim:        21144 kB
  KernelStack:        1176 kB
  PageTables:        11832 kB
  NFS_Unstable:          0 kB
  Bounce:                0 kB
  WritebackTmp:          0 kB
  CommitLimit:     3830632 kB
  Committed_AS:    6861504 kB
  VmallocTotal:   34359738367 kB
  VmallocUsed:       19132 kB
  VmallocChunk:   34359707084 kB
  HardwareCorrupted:     0 kB
  AnonHugePages:   4329472 kB
  HugePages_Total:       0
  HugePages_Free:        0
  HugePages_Rsvd:        0
  HugePages_Surp:        0
  Hugepagesize:       2048 kB
  DirectMap4k:       10240 kB
  DirectMap2M:     7985152 kB
  </code></pre>
  
  <p>Output of a <code>top</code>:</p>
  
  <pre><code>top - 06:04:57 up 16 days, 17:12,  1 user,  load average: 0.00, 0.04, 1.39
  Tasks:  76 total,   1 running,  75 sleeping,   0 stopped,   0 zombie
  %Cpu(s):  0.2 us,  0.3 sy,  0.0 ni, 99.5 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
  KiB Mem:   7661268 total,  7051140 used,   610128 free,     9772 buffers
  KiB Swap:        0 total,        0 used,        0 free.  2272468 cached Mem
  
    PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND                                                                     
  19958 elastic+  20   0 9746648 4.273g  22760 S   0.3 58.5   1:11.36 java                                                                        
  19970 root      20   0 1008496  89912   3876 S   0.3  1.2   0:07.13 node                                                                        
  19932 root      20   0   28256   6728   6496 S   0.0  0.1   0:01.29 systemd-journal                                                             
      1 root      20   0  110052   3316   1596 S   0.0  0.0   1:45.84 systemd                                                                     
  19856 root      20   0   16136   2100   1460 S   0.0  0.0   0:02.05 bash                                                                        
  19851 root      20   0   82472   2040   1136 S   0.0  0.0   0:01.39 sshd                                                                        
    245 haproxy   20   0   28872   1996    624 S   0.0  0.0   4:01.48 haproxy                                                                     
  19854 root      20   0  157700   1980      0 S   0.0  0.0   0:00.00 (sd-pam)                                                                    
    244 haproxy   20   0   28596   1728    768 S   0.0  0.0   0:00.00 haproxy                                                                     
    221 systemd+  20   0   23496   1476   1056 S   0.0  0.0   0:04.99 systemd-network                                                             
  20197 root      20   0   17544   1464   1064 R   0.0  0.0   0:00.02 top                                                                         
    149 root      20   0   31016   1456    768 S   0.0  0.0   0:00.10 systemd-udevd                                                               
    233 root      20   0   39980   1448    860 S   0.0  0.0   0:01.03 sshd                                                                        
    217 root      20   0   13204   1344    796 S   0.0  0.0   0:02.55 crond                                                                       
    219 dbus      20   0   24748   1236    900 S   0.0  0.0   0:18.18 dbus-daemon                                                                 
    229 systemd+  20   0   18164   1196    980 S   0.0  0.0   0:00.92 systemd-resolve                                                             
  19853 root      20   0   26764   1168    700 S   0.0  0.0   0:00.01 systemd                                                                     
    218 root      20   0   15252   1144    876 S   0.0  0.0   0:07.26 systemd-logind                                                              
    194 root      16  -4  105484   1136    764 S   0.0  0.0   0:03.15 auditd                                                                      
    214 root      20   0   19240   1024    776 S   0.0  0.0   1:08.62 irqbalance                                                                  
    226 root      20   0    8124    820    700 S   0.0  0.0   0:00.00 agetty                                                                      
    225 root      20   0    8124    816    700 S   0.0  0.0   0:00.00 agetty                                                                      
    224 root      20   0    8124    792    680 S   0.0  0.0   0:00.01 agetty                                                                      
    242 root      20   0   23896    624    292 S   0.0  0.0   0:00.00 haproxy-systemd                                                             
    213 root      20   0    9324    612    420 S   0.0  0.0   9:44.04 rngd                                                                        
      2 root      20   0       0      0      0 S   0.0  0.0   0:00.16 kthreadd                                                                    
      3 root      20   0       0      0      0 S   0.7  0.0  65:18.76 ksoftirqd/0                                                                 
      5 root       0 -20       0      0      0 S   0.0  0.0   0:00.00 kworker/0:0H                                                                
      7 root      20   0       0      0      0 S   0.0  0.0  10:08.87 rcu_sched                                                                   
      8 root      20   0       0      0      0 S   0.0  0.0   0:00.00 rcu_bh                                                                      
      9 root      rt   0       0      0      0 S   0.0  0.0   0:00.36 migration/0                                                                 
     10 root      rt   0       0      0      0 S   0.0  0.0   0:01.17 watchdog/0                                                                  
     11 root      rt   0       0      0      0 S   0.0  0.0   0:00.91 watchdog/1                                                                  
     12 root      rt   0       0      0      0 S   0.0  0.0   0:00.37 migration/1      
  </code></pre>