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Old 10-09-2009, 08:38 AM   #1
amit shukla
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kernel 2.6.9-89.0.9.ELsmp related question - drop_caches

We are currently on the kernel version 2.6.9-89.0.9.ELsmp and we have seen cases of cache usage going very high.(almost till the level of total available memory)
To solve this issue we used the following command:

echo 3 > /proc/sys/vm/drop_caches

The result is that it drops clean caches, dentries and inodes from memory, causing that memory to become free.

The man pages on the proc displays that drop_caches is available since the kernel version 2.6.16.
refert to the url below :

However as stated earlier drop_caches works on kernel version 2.6.9-89.0.9.ELsmp as well.
How can it be confirmed if its a safe option to use drop_caches in 2.6.9-89.0.9.ELsmp or
is any other source from where this can be verified ?

Thanks in advance.
Old 10-10-2009, 05:57 PM   #2
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The cache is designed to use free memory - that's how it provides a benefit. If you do a lot of I/O, it's not unusual for it to use "all" the memory. So you may be worrying about nothing.
Or you may actually have a problem - we can't tell from your post.

Patch sets are developed and tested prior to merging into the mainline kernel. Sometimes the major resellers roll patches they consider useful back into their user base. RH has a history of developing functions like this. Why would you think this is any less "safe" than any other patch ?.
With EL you should be on a support contract - you could always ask RH.
Old 10-10-2009, 09:22 PM   #3
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I don't remember the kernel level for RHEL4.6. I do however, have a painful memory of 'echo 3 > /proc/sys/vm/drop_caches' hanging a production server... So I'd be cautious.

I concur with syg00. Cache going high and using all available memory is not necessarily bad. Actually it is good most of the time. If you start swapping heavily (major faults) and your cache is not trimmed back, then you might want to look at 'swappiness' (/proc/sys/vm/swappiness). Although in my experience is it doesn't work very effectively on our systems (RHEL5.3). Just MHO.


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