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Why does Red Hat Linux report less free memory on the system than is actually available?

Hello,

I have a Dell R300 Ubuntu 9.10 box with 4GB of memory. All I'm running on there is haproxy, nagios and postfix yet there is ~2.7GB of memory being consumed. I've run ps and I can't get the sums to add up. Could anyone shed any light on where all the memory is being used?

Cheers, Mark

$ sudo free -m
             total       used       free     shared    buffers     cached
Mem:          3957       2746       1211          0        169       2320
-/+ buffers/cache:        256       3701
Swap:         6212          0       6212

Sorry for pasting all of ps' output but I'm keen to get to the bottom of this.

$ sudo ps aux
[sudo] password for mark: 
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root         1  0.0  0.0  19320  1656 ?        Ss   May20   0:05 /sbin/init
root         2  0.0  0.0      0     0 ?        S<   May20   0:00 [kthreadd]
root         3  0.0  0.0      0     0 ?        S<   May20   0:00 [migration/0]
root         4  0.0  0.0      0     0 ?        S<   May20   0:16 [ksoftirqd/0]
root         5  0.0  0.0      0     0 ?        S<   May20   0:00 [watchdog/0]
root         6  0.0  0.0      0     0 ?        S<   May20   0:03 [migration/1]
root         7  0.0  0.0      0     0 ?        S<   May20   3:10 [ksoftirqd/1]
root         8  0.0  0.0      0     0 ?        S<   May20   0:00 [watchdog/1]
root         9  0.0  0.0      0     0 ?        S<   May20   0:00 [migration/2]
root        10  0.0  0.0      0     0 ?        S<   May20   0:19 [ksoftirqd/2]
root        11  0.0  0.0      0     0 ?        S<   May20   0:00 [watchdog/2]
root        12  0.0  0.0      0     0 ?        S<   May20   0:01 [migration/3]
root        13  0.0  0.0      0     0 ?        S<   May20   0:41 [ksoftirqd/3]
root        14  0.0  0.0      0     0 ?        S<   May20   0:00 [watchdog/3]
root        15  0.0  0.0      0     0 ?        S<   May20   0:03 [events/0]
root        16  0.0  0.0      0     0 ?        S<   May20   0:10 [events/1]
root        17  0.0  0.0      0     0 ?        S<   May20   0:08 [events/2]
root        18  0.0  0.0      0     0 ?        S<   May20   0:08 [events/3]
root        19  0.0  0.0      0     0 ?        S<   May20   0:00 [cpuset]
root        20  0.0  0.0      0     0 ?        S<   May20   0:00 [khelper]
root        21  0.0  0.0      0     0 ?        S<   May20   0:00 [netns]
root        22  0.0  0.0      0     0 ?        S<   May20   0:00 [async/mgr]
root        23  0.0  0.0      0     0 ?        S<   May20   0:00 [kintegrityd/0]
root        24  0.0  0.0      0     0 ?        S<   May20   0:00 [kintegrityd/1]
root        25  0.0  0.0      0     0 ?        S<   May20   0:00 [kintegrityd/2]
root        26  0.0  0.0      0     0 ?        S<   May20   0:00 [kintegrityd/3]
root        27  0.0  0.0      0     0 ?        S<   May20   0:00 [kblockd/0]
root        28  0.0  0.0      0     0 ?        S<   May20   0:01 [kblockd/1]
root        29  0.0  0.0      0     0 ?        S<   May20   0:04 [kblockd/2]
root        30  0.0  0.0      0     0 ?        S<   May20   0:02 [kblockd/3]
root        31  0.0  0.0      0     0 ?        S<   May20   0:00 [kacpid]
root        32  0.0  0.0      0     0 ?        S<   May20   0:00 [kacpi_notify]
root        33  0.0  0.0      0     0 ?        S<   May20   0:00 [kacpi_hotplug]
root        34  0.0  0.0      0     0 ?        S<   May20   0:00 [ata/0]
root        35  0.0  0.0      0     0 ?        S<   May20   0:00 [ata/1]
root        36  0.0  0.0      0     0 ?        S<   May20   0:00 [ata/2]
root        37  0.0  0.0      0     0 ?        S<   May20   0:00 [ata/3]
root        38  0.0  0.0      0     0 ?        S<   May20   0:00 [ata_aux]
root        39  0.0  0.0      0     0 ?        S<   May20   0:00 [ksuspend_usbd]
root        40  0.0  0.0      0     0 ?        S<   May20   0:00 [khubd]
root        41  0.0  0.0      0     0 ?        S<   May20   0:00 [kseriod]
root        42  0.0  0.0      0     0 ?        S<   May20   0:00 [kmmcd]
root        43  0.0  0.0      0     0 ?        S<   May20   0:00 [bluetooth]
root        44  0.0  0.0      0     0 ?        S    May20   0:00 [khungtaskd]
root        45  0.0  0.0      0     0 ?        S    May20   0:00 [pdflush]
root        46  0.0  0.0      0     0 ?        S    May20   0:09 [pdflush]
root        47  0.0  0.0      0     0 ?        S<   May20   0:00 [kswapd0]
root        48  0.0  0.0      0     0 ?        S<   May20   0:00 [aio/0]
root        49  0.0  0.0      0     0 ?        S<   May20   0:00 [aio/1]
root        50  0.0  0.0      0     0 ?        S<   May20   0:00 [aio/2]
root        51  0.0  0.0      0     0 ?        S<   May20   0:00 [aio/3]
root        52  0.0  0.0      0     0 ?        S<   May20   0:00 [ecryptfs-kthrea]
root        53  0.0  0.0      0     0 ?        S<   May20   0:00 [crypto/0]
root        54  0.0  0.0      0     0 ?        S<   May20   0:00 [crypto/1]
root        55  0.0  0.0      0     0 ?        S<   May20   0:00 [crypto/2]
root        56  0.0  0.0      0     0 ?        S<   May20   0:00 [crypto/3]
root        70  0.0  0.0      0     0 ?        S<   May20   0:00 [scsi_eh_0]
root        71  0.0  0.0      0     0 ?        S<   May20   0:00 [scsi_eh_1]
root        74  0.0  0.0      0     0 ?        S<   May20   0:00 [scsi_eh_2]
root        75  0.0  0.0      0     0 ?        S<   May20   0:00 [scsi_eh_3]
root        82  0.0  0.0      0     0 ?        S<   May20   0:00 [kstriped]
root        83  0.0  0.0      0     0 ?        S<   May20   0:00 [kmpathd/0]
root        84  0.0  0.0      0     0 ?        S<   May20   0:00 [kmpathd/1]
root        85  0.0  0.0      0     0 ?        S<   May20   0:00 [kmpathd/2]
root        86  0.0  0.0      0     0 ?        S<   May20   0:00 [kmpathd/3]
root        87  0.0  0.0      0     0 ?        S<   May20   0:00 [kmpath_handlerd]
root        88  0.0  0.0      0     0 ?        S<   May20   0:00 [ksnapd]
root        89  0.0  0.0      0     0 ?        S<   May20   0:00 [kondemand/0]
root        90  0.0  0.0      0     0 ?        S<   May20   0:00 [kondemand/1]
root        91  0.0  0.0      0     0 ?        S<   May20   0:00 [kondemand/2]
root        92  0.0  0.0      0     0 ?        S<   May20   0:00 [kondemand/3]
root        93  0.0  0.0      0     0 ?        S<   May20   0:00 [kconservative/0]
root        94  0.0  0.0      0     0 ?        S<   May20   0:00 [kconservative/1]
root        95  0.0  0.0      0     0 ?        S<   May20   0:00 [kconservative/2]
root        96  0.0  0.0      0     0 ?        S<   May20   0:00 [kconservative/3]
root        97  0.0  0.0      0     0 ?        S<   May20   0:00 [krfcommd]
root       315  0.0  0.0      0     0 ?        S<   May20   0:09 [mpt_poll_0]
root       317  0.0  0.0      0     0 ?        S<   May20   0:00 [mpt/0]
root       547  0.0  0.0      0     0 ?        S<   May20   0:00 [scsi_eh_4]
root       587  0.0  0.0      0     0 ?        S<   May20   0:11 [kjournald2]
root       636  0.0  0.0  12748   860 ?        S    May20   0:00 upstart-udev-bridge --daemon
root       657  0.0  0.0  17064   924 ?        S<s  May20   0:00 udevd --daemon
root       666  0.0  0.0   8192   612 ?        Ss   May20   0:00 dd bs=1 if=/proc/kmsg of=/var/run/rsyslog/kmsg
root       774  0.0  0.0  17060   888 ?        S<   May20   0:00 udevd --daemon
root       775  0.0  0.0  17060   888 ?        S<   May20   0:00 udevd --daemon
syslog     825  0.0  0.0 191696  1988 ?        Sl   May20   0:31 rsyslogd -c4
root       839  0.0  0.0      0     0 ?        S<   May20   0:00 [edac-poller]
root       870  0.0  0.0      0     0 ?        S<   May20   0:00 [kpsmoused]
root      1006  0.0  0.0   5988   604 tty4     Ss+  May20   0:00 /sbin/getty -8 38400 tty4
root      1008  0.0  0.0   5988   604 tty5     Ss+  May20   0:00 /sbin/getty -8 38400 tty5
root      1015  0.0  0.0   5988   604 tty2     Ss+  May20   0:00 /sbin/getty -8 38400 tty2
root      1016  0.0  0.0   5988   608 tty3     Ss+  May20   0:00 /sbin/getty -8 38400 tty3
root      1018  0.0  0.0   5988   604 tty6     Ss+  May20   0:00 /sbin/getty -8 38400 tty6
daemon    1025  0.0  0.0  16512   472 ?        Ss   May20   0:00 atd
root      1026  0.0  0.0  18708  1000 ?        Ss   May20   0:03 cron
root      1052  0.0  0.0  49072  1252 ?        Ss   May20   0:25 /usr/sbin/sshd
root      1084  0.0  0.0   5988   604 tty1     Ss+  May20   0:00 /sbin/getty -8 38400 tty1
root      6320  0.0  0.0  19440   956 ?        Ss   May21   0:00 /usr/sbin/xinetd -pidfile /var/run/xinetd.pid -stayalive -inetd_compat -inetd_ipv6
nagios    8197  0.0  0.0  27452  1696 ?        SNs  May21   2:57 /usr/sbin/nagios3 -d /etc/nagios3/nagios.cfg
root     10882  0.1  0.0  70280  3104 ?        Ss   10:30   0:00 sshd: mark [priv]
mark     10934  0.0  0.0  70432  1776 ?        S    10:30   0:00 sshd: mark@pts/0 
mark     10935  1.4  0.1  21572  4336 pts/0    Ss   10:30   0:00 -bash
root     10953  1.0  0.0  15164  1136 pts/0    R+   10:30   0:00 ps aux
haproxy  12738  0.0  0.0  17208   992 ?        Ss   Jun08   0:49 /usr/sbin/haproxy -f /etc/haproxy/haproxy.cfg
root     23953  0.0  0.0  37012  2192 ?        Ss   Jun04   0:03 /usr/lib/postfix/master
postfix  23955  0.0  0.0  39232  2356 ?        S    Jun04   0:00 qmgr -l -t fifo -u
postfix  32603  0.0  0.0  39072  2132 ?        S    09:05   0:00 pickup -l -t fifo -u -c

Here's meminfo:

$ cat /proc/meminfo
MemTotal:        4052852 kB
MemFree:         1240488 kB
Buffers:          173172 kB
Cached:          2376420 kB
SwapCached:            0 kB
Active:          1479288 kB
Inactive:        1081876 kB
Active(anon):      11792 kB
Inactive(anon):        0 kB
Active(file):    1467496 kB
Inactive(file):  1081876 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:       6361700 kB
SwapFree:        6361700 kB
Dirty:                44 kB
Writeback:             0 kB
AnonPages:         11568 kB
Mapped:             5844 kB
Slab:             155032 kB
SReclaimable:     145804 kB
SUnreclaim:         9228 kB
PageTables:         1592 kB
NFS_Unstable:          0 kB
Bounce:                0 kB
WritebackTmp:          0 kB
CommitLimit:     8388124 kB
Committed_AS:      51732 kB
VmallocTotal:   34359738367 kB
VmallocUsed:      282604 kB
VmallocChunk:   34359453499 kB
HugePages_Total:       0
HugePages_Free:        0
HugePages_Rsvd:        0
HugePages_Surp:        0
Hugepagesize:       2048 kB
DirectMap4k:        6784 kB
DirectMap2M:     4182016 kB

Here's slabinfo:

$ cat /proc/slabinfo
slabinfo - version: 2.1
# name            <active_objs> <num_objs> <objsize> <objperslab> <pagesperslab> : tunables <limit> <batchcount> <sharedfactor> : slabdata <active_slabs> <num_slabs> <sharedavail>
ip6_dst_cache         50     50    320   25    2 : tunables    0    0    0 : slabdata      2      2      0
UDPLITEv6              0      0    960   17    4 : tunables    0    0    0 : slabdata      0      0      0
UDPv6                 68     68    960   17    4 : tunables    0    0    0 : slabdata      4      4      0
tw_sock_TCPv6          0      0    320   25    2 : tunables    0    0    0 : slabdata      0      0      0
TCPv6                 72     72   1792   18    8 : tunables    0    0    0 : slabdata      4      4      0
dm_raid1_read_record      0      0   1064   30    8 : tunables    0    0    0 : slabdata      0      0      0
kcopyd_job             0      0    368   22    2 : tunables    0    0    0 : slabdata      0      0      0
dm_uevent              0      0   2608   12    8 : tunables    0    0    0 : slabdata      0      0      0
dm_rq_target_io        0      0    376   21    2 : tunables    0    0    0 : slabdata      0      0      0
uhci_urb_priv          0      0     56   73    1 : tunables    0    0    0 : slabdata      0      0      0
cfq_queue              0      0    168   24    1 : tunables    0    0    0 : slabdata      0      0      0
mqueue_inode_cache     18     18    896   18    4 : tunables    0    0    0 : slabdata      1      1      0
fuse_request           0      0    632   25    4 : tunables    0    0    0 : slabdata      0      0      0
fuse_inode             0      0    768   21    4 : tunables    0    0    0 : slabdata      0      0      0
ecryptfs_inode_cache      0      0   1024   16    4 : tunables    0    0    0 : slabdata      0      0      0
hugetlbfs_inode_cache     26     26    608   26    4 : tunables    0    0    0 : slabdata      1      1      0
journal_handle       680    680     24  170    1 : tunables    0    0    0 : slabdata      4      4      0
journal_head         144    144    112   36    1 : tunables    0    0    0 : slabdata      4      4      0
revoke_table         256    256     16  256    1 : tunables    0    0    0 : slabdata      1      1      0
revoke_record        512    512     32  128    1 : tunables    0    0    0 : slabdata      4      4      0
ext4_inode_cache   53306  53424    888   18    4 : tunables    0    0    0 : slabdata   2968   2968      0
ext4_free_block_extents    292    292     56   73    1 : tunables    0    0    0 : slabdata      4      4      0
ext4_alloc_context    112    112    144   28    1 : tunables    0    0    0 : slabdata      4      4      0
ext4_prealloc_space    156    156    104   39    1 : tunables    0    0    0 : slabdata      4      4      0
ext4_system_zone       0      0     40  102    1 : tunables    0    0    0 : slabdata      0      0      0
ext2_inode_cache       0      0    776   21    4 : tunables    0    0    0 : slabdata      0      0      0
ext3_inode_cache       0      0    784   20    4 : tunables    0    0    0 : slabdata      0      0      0
ext3_xattr             0      0     88   46    1 : tunables    0    0    0 : slabdata      0      0      0
dquot                  0      0    256   16    1 : tunables    0    0    0 : slabdata      0      0      0
shmem_inode_cache    606    620    800   20    4 : tunables    0    0    0 : slabdata     31     31      0
pid_namespace          0      0   2112   15    8 : tunables    0    0    0 : slabdata      0      0      0
UDP-Lite               0      0    832   19    4 : tunables    0    0    0 : slabdata      0      0      0
RAW                  183    210    768   21    4 : tunables    0    0    0 : slabdata     10     10      0
UDP                   76     76    832   19    4 : tunables    0    0    0 : slabdata      4      4      0
tw_sock_TCP           80     80    256   16    1 : tunables    0    0    0 : slabdata      5      5      0
TCP                   81    114   1664   19    8 : tunables    0    0    0 : slabdata      6      6      0
blkdev_integrity     144    144    112   36    1 : tunables    0    0    0 : slabdata      4      4      0
blkdev_queue          64     64   2024   16    8 : tunables    0    0    0 : slabdata      4      4      0
blkdev_requests      120    120    336   24    2 : tunables    0    0    0 : slabdata      5      5      0
fsnotify_event       156    156    104   39    1 : tunables    0    0    0 : slabdata      4      4      0
bip-256                7      7   4224    7    8 : tunables    0    0    0 : slabdata      1      1      0
bip-128                0      0   2176   15    8 : tunables    0    0    0 : slabdata      0      0      0
bip-64                 0      0   1152   28    8 : tunables    0    0    0 : slabdata      0      0      0
bip-16                84     84    384   21    2 : tunables    0    0    0 : slabdata      4      4      0
sock_inode_cache     224    276    704   23    4 : tunables    0    0    0 : slabdata     12     12      0
file_lock_cache       88     88    184   22    1 : tunables    0    0    0 : slabdata      4      4      0
net_namespace          0      0   1920   17    8 : tunables    0    0    0 : slabdata      0      0      0
Acpi-ParseExt        640    672     72   56    1 : tunables    0    0    0 : slabdata     12     12      0
taskstats             48     48    328   24    2 : tunables    0    0    0 : slabdata      2      2      0
proc_inode_cache    1613   1750    640   25    4 : tunables    0    0    0 : slabdata     70     70      0
sigqueue             100    100    160   25    1 : tunables    0    0    0 : slabdata      4      4      0
radix_tree_node    22443  22475    560   29    4 : tunables    0    0    0 : slabdata    775    775      0
bdev_cache            72     72    896   18    4 : tunables    0    0    0 : slabdata      4      4      0
sysfs_dir_cache     9866   9894     80   51    1 : tunables    0    0    0 : slabdata    194    194      0
inode_cache         2268   2268    592   27    4 : tunables    0    0    0 : slabdata     84     84      0
dentry            285907 286062    192   21    1 : tunables    0    0    0 : slabdata  13622  13622      0
buffer_head       256447 257472    112   36    1 : tunables    0    0    0 : slabdata   7152   7152      0
vm_area_struct      1469   1541    176   23    1 : tunables    0    0    0 : slabdata     67     67      0
mm_struct             82     95    832   19    4 : tunables    0    0    0 : slabdata      5      5      0
files_cache          104    161    704   23    4 : tunables    0    0    0 : slabdata      7      7      0
signal_cache         163    187    960   17    4 : tunables    0    0    0 : slabdata     11     11      0
sighand_cache        145    165   2112   15    8 : tunables    0    0    0 : slabdata     11     11      0
task_xstate          118    140    576   28    4 : tunables    0    0    0 : slabdata      5      5      0
task_struct          128    165   5808    5    8 : tunables    0    0    0 : slabdata     33     33      0
anon_vma             731    896     32  128    1 : tunables    0    0    0 : slabdata      7      7      0
shared_policy_node     85     85     48   85    1 : tunables    0    0    0 : slabdata      1      1      0
numa_policy          170    170     24  170    1 : tunables    0    0    0 : slabdata      1      1      0
idr_layer_cache      240    240    544   30    4 : tunables    0    0    0 : slabdata      8      8      0
kmalloc-8192          27     32   8192    4    8 : tunables    0    0    0 : slabdata      8      8      0
kmalloc-4096         291    344   4096    8    8 : tunables    0    0    0 : slabdata     43     43      0
kmalloc-2048         225    240   2048   16    8 : tunables    0    0    0 : slabdata     15     15      0
kmalloc-1024         366    432   1024   16    4 : tunables    0    0    0 : slabdata     27     27      0
kmalloc-512          536    544    512   16    2 : tunables    0    0    0 : slabdata     34     34      0
kmalloc-256          406    528    256   16    1 : tunables    0    0    0 : slabdata     33     33      0
kmalloc-128          503    576    128   32    1 : tunables    0    0    0 : slabdata     18     18      0
kmalloc-64          3467   3712     64   64    1 : tunables    0    0    0 : slabdata     58     58      0
kmalloc-32          1520   1920     32  128    1 : tunables    0    0    0 : slabdata     15     15      0
kmalloc-16          3547   3840     16  256    1 : tunables    0    0    0 : slabdata     15     15      0
kmalloc-8           4607   4608      8  512    1 : tunables    0    0    0 : slabdata      9      9      0
kmalloc-192         4620   5313    192   21    1 : tunables    0    0    0 : slabdata    253    253      0
kmalloc-96          1780   1848     96   42    1 : tunables    0    0    0 : slabdata     44     44      0
kmem_cache_node        0      0     64   64    1 : tunables    0    0    0 : slabdata      0      0      0
share|improve this question

marked as duplicate by ThatGraemeGuy, Dennis Williamson, wzzrd, Kyle Brandt, Warner Jun 17 '10 at 13:07

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

3 Answers 3

up vote 9 down vote accepted

Actually, you are misinterpreting the output of free.

$ sudo free -m
             total       used       free     shared    buffers     cached
Mem:          3957       2746       1211          0        169       2320
-/+ buffers/cache:        256       3701
Swap:         6212          0       6212

This tells you you have 3957 megabytes of memory, of which 1211 is "instantly free", and 2746 is "in use right now".

But of those 2746 megabytes, 2320 megabytes are used by the linux cache because they weren't being used otherwise - linux optimizes this and decides to use the unused RAM to speed up access to frequently requested files etc.

Your actual amount of free RAM is this 2320+1211 = 3531. Sounds better, doesn't it?

Don't worry about the memory being used by the cache - when an application demands more memory, linux will reduce the cache and give it to that application. What is important is that your SWAP file (6212) is not being used - this is good.

share|improve this answer
    
Fantastic answer, thank you for clearing that up for me! –  Mark L Jun 17 '10 at 10:01

Why are you worried about the amount of memory listed as free? Are you having a problem?

Any modern operating system will try and find a use for as much of your installed memory as it can to improve the responsiveness of the OS and your applications and it will free this memory on request.

This is a good thing - you presumably installed the memory in the hope that the OS would find some kind of use for it? Very large amounts of memory that is never used is arguably memory you wasted money on.

share|improve this answer
    
I was concerned as I thought the system would just start swapping soon. Knowing now that Linux uses all the memory it can for caches which will expire if need be is comforting to know. –  Mark L Jun 17 '10 at 10:09

you check this using small code which our team has created

http://studyhat.blogspot.com/2009/10/linux-memory-use.html

may be help you

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Try a <pre> tag. –  Tobu Jun 17 '10 at 10:30
    
next time i'll use thanks –  Rajat Jun 17 '10 at 10:34

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