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I've noticed high memory usage on the server. Details:

OS: CentOS 6.3 - x86_64

Web server: Nginx with PHP-FPM

The server is generating PDF documents so the traffic is minimum.

top:

# top -b -n 1 -a
top - 10:04:51 up 21 days, 18:57,  1 user,  load average: 0.00, 0.00, 0.00
Tasks:  92 total,   1 running,  91 sleeping,   0 stopped,   0 zombie
Cpu(s):  0.3%us,  0.2%sy,  0.0%ni, 99.6%id,  0.0%wa,  0.0%hi,  0.0%si,  0.0%st
Mem:   3923092k total,  3720380k used,   202712k free,   133904k buffers
Swap:  4194296k total,       12k used,  4194284k free,   147404k cached

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                                                     
15855 www-data  20   0  199m 4952 2128 S  0.0  0.1   0:00.06 php-fpm                                                                                     
15853 www-data  20   0  199m 4940 2028 S  0.0  0.1   0:00.06 php-fpm                                                                                     
15850 www-data  20   0  199m 4928 2020 S  0.0  0.1   0:00.05 php-fpm                                                                                     
15851 www-data  20   0  199m 4888 2020 S  0.0  0.1   0:00.06 php-fpm                                                                                     
15852 www-data  20   0  199m 4852 2020 S  0.0  0.1   0:00.06 php-fpm                                                                                     
15857 www-data  20   0  198m 4716 2020 S  0.0  0.1   0:00.06 php-fpm                                                                                     
17553 root      20   0 97816 3860 2924 S  0.0  0.1   0:00.03 sshd                                                                                        
15849 root      20   0  198m 3460 1072 S  0.0  0.1   0:00.12 php-fpm                                                                                     
13441 nginx     20   0 65608 2968 1604 S  0.0  0.1   0:02.06 nginx                                                                                       
13440 nginx     20   0 65608 2964 1600 S  0.0  0.1   0:01.87 nginx                                                                                       
17561 root      20   0  105m 1944 1488 S  0.0  0.0   0:00.01 bash                                                                                        
 1150 xfs       20   0 20980 1784  704 S  0.0  0.0   0:00.13 xfs                                                                                         
15863 root      20   0  179m 1424 1028 S  0.0  0.0   0:00.00 rsyslogd                                                                                    
    1 root      20   0 19224 1360 1088 S  0.0  0.0   0:17.96 init                                                                                        
 1201 nrpe      20   0 40928 1288  704 S  0.0  0.0   3:57.64 nrpe                                                                                        
13226 root      20   0  114m 1216  612 S  0.0  0.0   0:00.01 crond                                                                                       
 6691 root      20   0 64068 1156  488 S  0.0  0.0   0:09.59 sshd                                                                                        
13439 root      20   0 65104 1128  292 S  0.0  0.0   0:00.00 nginx                                                                                       
19026 root      20   0 15040 1116  844 R  0.0  0.0   0:00.00 top                                                                                         
  451 root      16  -4 11052 1096  316 S  0.0  0.0   0:00.02 udevd                                                                                       
 1174 root      18  -2 11048 1064  288 S  0.0  0.0   0:00.00 udevd                                                                                       
 1175 root      18  -2 11048 1064  288 S  0.0  0.0   0:00.00 udevd                                                                                       
 1065 root      16  -4 93168  824  560 S  0.0  0.0   0:16.00 auditd                                                                                      
 1165 root      20   0  4056  564  480 S  0.0  0.0   0:00.00 mingetty                                                                                    
 1167 root      20   0  4056  564  480 S  0.0  0.0   0:00.00 mingetty                                                                                    
 1169 root      20   0  4056  564  480 S  0.0  0.0   0:00.00 mingetty                                                                                    
 1171 root      20   0  4056  564  480 S  0.0  0.0   0:00.00 mingetty                                                                                    
 1163 root      20   0  4056  560  480 S  0.0  0.0   0:00.00 mingetty                                                                                    
 1176 root      20   0  4056  560  480 S  0.0  0.0   0:00.00 mingetty                                                                                    
    2 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthreadd                                                                                    
    3 root      RT   0     0    0    0 S  0.0  0.0   0:11.75 migration/0                                                                                 
    4 root      20   0     0    0    0 S  0.0  0.0  44:30.28 ksoftirqd/0                                                                                 
    5 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/0                                                                                 
    6 root      RT   0     0    0    0 S  0.0  0.0   0:03.51 watchdog/0                                                                                  
    7 root      RT   0     0    0    0 S  0.0  0.0   0:11.63 migration/1                                                                                 
    8 root      RT   0     0    0    0 S  0.0  0.0   0:00.00 migration/1                                                                                 
    9 root      20   0     0    0    0 S  0.0  0.0  11:35.50 ksoftirqd/1                                                                                 
   10 root      RT   0     0    0    0 S  0.0  0.0   0:03.34 watchdog/1                                                                                  
   11 root      20   0     0    0    0 S  0.0  0.0   1:36.68 events/0                                                                                    
   12 root      20   0     0    0    0 S  0.0  0.0   1:50.57 events/1                                                                                    
   13 root      20   0     0    0    0 S  0.0  0.0   0:00.00 cgroup                                                                                      
   14 root      20   0     0    0    0 S  0.0  0.0   0:00.00 khelper                                                                                     
   15 root      20   0     0    0    0 S  0.0  0.0   0:00.00 netns                                                                                       
   16 root      20   0     0    0    0 S  0.0  0.0   0:00.00 async/mgr                                                                                   
   17 root      20   0     0    0    0 S  0.0  0.0   0:00.00 pm                                                                                          
   18 root      20   0     0    0    0 S  0.0  0.0   0:07.86 sync_supers                                                                                 
   19 root      20   0     0    0    0 S  0.0  0.0   0:10.38 bdi-default                                                                                 
   20 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kintegrityd/0                                                                               
   21 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kintegrityd/1                                                                               
   22 root      20   0     0    0    0 S  0.0  0.0   0:04.35 kblockd/0                                                                                   
   23 root      20   0     0    0    0 S  0.0  0.0   0:04.18 kblockd/1                                                                                   
   24 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kacpid                                                                                      
   25 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kacpi_notify                                                                                
   26 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kacpi_hotplug                                                                               
   27 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ata/0                                                                                       
   28 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ata/1                                                                                       
   29 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ata_aux                                                                                     
   30 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ksuspend_usbd                                                                               
   31 root      20   0     0    0    0 S  0.0  0.0   0:00.00 khubd                                                                                       
   32 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kseriod                                                                                     
   33 root      20   0     0    0    0 S  0.0  0.0   0:00.00 md/0                                                                                        
   34 root      20   0     0    0    0 S  0.0  0.0   0:00.00 md/1                                                                                        
   35 root      20   0     0    0    0 S  0.0  0.0   0:00.00 md_misc/0                                                                                   
   36 root      20   0     0    0    0 S  0.0  0.0   0:00.00 md_misc/1                                                                                   
   37 root      20   0     0    0    0 S  0.0  0.0   0:00.48 khungtaskd                                                                                  
   38 root      20   0     0    0    0 S  0.0  0.0   1:07.52 kswapd0                                                                                     
   39 root      25   5     0    0    0 S  0.0  0.0   0:00.00 ksmd                                                                                        
   40 root      39  19     0    0    0 S  0.0  0.0   0:22.00 khugepaged                                                                                  
   41 root      20   0     0    0    0 S  0.0  0.0   0:00.00 aio/0                                                                                       
   42 root      20   0     0    0    0 S  0.0  0.0   0:00.00 aio/1                                                                                       
   43 root      20   0     0    0    0 S  0.0  0.0   0:00.00 crypto/0                                                                                    
   44 root      20   0     0    0    0 S  0.0  0.0   0:00.00 crypto/1                                                                                    
   49 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthrotld/0                                                                                  
   50 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kthrotld/1                                                                                  
   52 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kpsmoused                                                                                   
   53 root      20   0     0    0    0 S  0.0  0.0   0:00.00 usbhid_resumer                                                                              
   83 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kstriped                                                                                    
  233 root      20   0     0    0    0 S  0.0  0.0   0:00.00 scsi_eh_0                                                                                   
  234 root      20   0     0    0    0 S  0.0  0.0   0:00.00 scsi_eh_1                                                                                   
  321 root      20   0     0    0    0 S  0.0  0.0   0:00.00 virtio-blk                                                                                  
  359 root      20   0     0    0    0 S  0.0  0.0   0:03.24 kdmflush                                                                                    
  360 root      20   0     0    0    0 S  0.0  0.0   0:00.00 kdmflush                                                                                    
  380 root      20   0     0    0    0 S  0.0  0.0   0:20.64 jbd2/dm-0-8                                                                                 
  381 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ext4-dio-unwrit                                                                             
  382 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ext4-dio-unwrit                                                                             
  694 root      20   0     0    0    0 S  0.0  0.0   0:00.00 vballoon                                                                                    
  697 root      20   0     0    0    0 S  0.0  0.0   0:00.00 virtio-net                                                                                  
  818 root      20   0     0    0    0 S  0.0  0.0   0:00.00 jbd2/vda1-8                                                                                 
  819 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ext4-dio-unwrit                                                                             
  820 root      20   0     0    0    0 S  0.0  0.0   0:00.00 ext4-dio-unwrit                                                                             
  851 root      20   0     0    0    0 S  0.0  0.0   0:06.96 kauditd                                                                                     
 1013 root      20   0     0    0    0 S  0.0  0.0   0:15.45 flush-253:0

ps:

# ps aux --sort -vsz | head
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
www-data 13213  0.0  0.1 204416  4772 ?        S    08:28   0:00 php-fpm: pool default
www-data 13214  0.0  0.1 204416  4776 ?        S    08:28   0:00 php-fpm: pool default
www-data 13215  0.0  0.1 204416  4832 ?        S    08:28   0:00 php-fpm: pool default
www-data 13216  0.0  0.1 204416  4776 ?        S    08:28   0:00 php-fpm: pool default
www-data 13218  0.0  0.1 204416  4956 ?        S    08:28   0:00 php-fpm: pool default

free:

#free -m
             total       used       free     shared    buffers     cached
Mem:          3831       3530        300          0        130        143
-/+ buffers/cache:       3256        574
Swap:         4095          0       4095

When I stooped Nginx, PHP-FPM the memory usage was still the same.

Could you help me to investigate what is consuming the memory on the system?

Regards

share|improve this question
    
Are you experiencing performance problems? If so, can you describe them? –  David Schwartz Sep 1 '12 at 8:23
    
Well the problem is that this server is almost idle and I'm just curious why the memory usage is so high. –  HTF Sep 1 '12 at 8:52
1  
I see you have about 3.5Gb free. Can you explain how do you see your system have a high memory usage ? You can look at my answer below. –  Gregory MOUSSAT Sep 11 '12 at 15:18

3 Answers 3

Your KVM-based virtual machine is reporting much more memory used than the running processes account for.

I would guess the most likely cause of this is that the host is using memory ballooning to overcommit memory usage on the host.

You won't have much control over this unless you also control the host. If you've leased this virtual server, check with the host regarding their memory overcommit policies.

share|improve this answer

You might also want to check for ramdisk mounts. A mount|grep tmpfs should show any. If there are large files in those directories, that might cause large RAM usage without it showing up in top, though I don't think it's possible for a ramdisk to take up all the memory.

share|improve this answer

You misread the output of free.
This command show your sever has 300 MiB free and 3256 MiB used for buffers and cache. So you only have 3831-3256=575 MiB used. Which is clearly not high usage.
Edit : I took the wrong numbers. This is 3530-3256=274MiB used by programs and kernel, without counting buffers and cache.

The memory used by buffers and cache will be released if any program needs it.
As your sever don't use lots of memory, the kernel takes advantage of it to store some datas which can be later discarded. When a program needs more memory, the kernel discard some cache to give this memory to the program.
The amount of really available memory is tuned by the kernel. If the I/O load is heavy, the kernel tend to keep more memory for cache.

share|improve this answer
    
You've read it backwards. He has 575 MB free, not used. –  Michael Hampton Sep 1 '12 at 15:09

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