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rhel not releasing chache memory for applications

Posted on 2014-03-25
Medium Priority
Last Modified: 2014-04-08
Hi All,

I have applications configured on RHEL 5.5 on VMware machine.

Recently migrated from Physical to Vmware

- Problem is coming that now application used all RAM fast and then application crashed (Only application) and to avoid that either we need to restart the application or echo 3 to drop caches.

This is really painful to do .

What i checked that when the memory utilizatio  reached 99% also then too it doesnt release the Cached memory for applications .

Please help why its happening and how to resolve this?

current swappiness value is : 60

Total RAM : 16 GB
Swap : 1 GB   ( It always free - never used)

Question by:apunkabollywood
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Author Comment

ID: 39953142
More info - Main application running is APACHE (HTTP) on it.

Author Comment

ID: 39954175
Memory info

MemTotal:     16436872 kB
MemFree:         89416 kB
Buffers:        381188 kB
Cached:       12983640 kB
SwapCached:          0 kB
Active:        3308144 kB
Inactive:     12825452 kB
HighTotal:           0 kB
HighFree:            0 kB
LowTotal:     16436872 kB
LowFree:         89416 kB
SwapTotal:     1048568 kB
SwapFree:      1048464 kB
Dirty:             176 kB
Writeback:           0 kB
AnonPages:     2768628 kB
Mapped:         141028 kB
Slab:           168112 kB
PageTables:      19964 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
CommitLimit:   9267004 kB
Committed_AS:  5366696 kB
VmallocTotal: 34359738367 kB
VmallocUsed:    264080 kB
VmallocChunk: 34359473527 kB
HugePages_Total:     0
HugePages_Free:      0
HugePages_Rsvd:      0
Hugepagesize:     2048 kB
LVL 20

Expert Comment

ID: 39954571
How do you know that cache is not being released?  Does Apache throw an "out of memory" error?

Out of interest, how much memory does the host machine have?
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LVL 35

Expert Comment

by:Seth Simmons
ID: 39954781
swappiness value 60, 1gb swap partition and not used doesn't sound much like an issue itself

the kernel will release cache as needed; won't automatically flush as you would manually to drop_caches

if the web application is crashing i would look at apache logs, any addins configured (php, etc.) and go from there - especially if apache processes are using that much memory.  the cache could be used by the application.

another thing to check is messages to see if the kernel is invoking OOM killer on apache.  i've seen this before with a badly configured application that causes a memory leak

Author Comment

ID: 39955167
These are the logs coming from Kernel:

kernel: ll header: ff:ff:ff:ff:ff:ff:00:15:60:8a:53:4f:08:00
kernel: martian source from, on dev eth3
kernel: ll header: ff:ff:ff:ff:ff:ff:00:15:60:8a:53:4f:08:00
kernel: martian source from on dev eth3
kernel: ll header: ff:ff:ff:ff:ff:ff:00:15:60:8a:53:4f:08:00
kernel: martian source from, on dev eth2
kernel: ll header: ff:ff:ff:ff:ff:ff:00:17:a4:3e:54:4f:08:00
 printk: 3 messages suppressed.

these are the logs i found nothing else
LVL 62

Expert Comment

ID: 39955233
redhat (or any other Linux) will use all free RAM for cache.
vm.min_free_kbytes can be set to keep as much free memory as you want
namely i'd suggest dropping VM memory to 8GB as only <4G is used at any time
(With RHEL6 where RAM hotplug works i'd say to enable that and start at 4G and size up asneeded)

and either set guest mem warning threshold to something more appropriate
or use vm.min_free_kbytes to be 5% or more of MemTotal: form /proc/meminfo

Actually Linux kernel will stop at 64M in free kbytes, so you can rise it. e.g for DB2 IBM recommends setting that to 6% of MemTotal...

Also feel free to discuss with vmware support why they never counted cache for free memory, when actually their balloon driver will discard cache just like another free RAM...

Author Comment

ID: 39955325
Hi Ghiest,

This is the current value on server:
 cat /proc/sys/vm/min_free_kbytes

and i have total ram of 16 GB and 1 GB swap

what you recommend?

and its  RedHat RHEL 5.5 x86_64

Author Comment

ID: 39955368
Actually problem is this we need to drop cache in every 5 hours because RAM used to get full in 5 hours - if we dont drop cache application need to be crashed and then restart the app again
LVL 20

Expert Comment

ID: 39955491
If dropping cache solves your problem (though I can't see why it helps), you could just add

    */3 * * * * echo 3 > /proc/sys/vm/drop_caches

to your crontab to drop cache every three hours.

Author Comment

ID: 39955506
Hi Simon - its a production server and also memory full in  hours  ...i think as per redhat too this is not recommended for production servers?
LVL 20

Expert Comment

ID: 39955547
Not ideal for production (disk access will spike after you do this), but if it keeps the server running, it's better than a crash.
LVL 20

Expert Comment

ID: 39955551
Is there anything relevant in the logs for the host machine?  If that starts to swap the VMs because *it* is running out of memory, that might cause a problem within the VMs.
LVL 88

Expert Comment

ID: 39955591
Maybe you haven't installed the VMware Tools?
LVL 62

Accepted Solution

gheist earned 2000 total points
ID: 39955735
You can rise the vm.min_free_kbytes to 6% of RAM (i.e approx 1G)
or at least as on RHEL6 hugeadm recommends on 16G system - to 65536
sysctl vm.min_free_kbytes=65536 >> /etc/sysctl.conf

Actually it is wrong probe from vmware.

Author Closing Comment

ID: 39985414
Thanks Ghiest for your support

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