On a Centos 7 server with 32GB RAM memory I am running a few programs namely MySQL, Apache2, PHP. Recently I wanted to verify the amount of ram left as I was planning on installing a few more programs, to my suprise the amount of memory was quiet low! After investigating I found over 20GB was being use by Slab. 2 days ago I dropped the cache's so the slab usage went down to 0 and slowly increased again. While monitoring it with a program I noticed the usage going up in a linear pattern. In the last 24 hours it increased with ~5200MB (Total increase of 13GB over 60 hours). The total data on the disk is under 40GB. The output of 'find /' is merely a few MB. It seems there are somehow a lot if dentries being cached?

I have posts saying NSS which came with curl was the cause. I checked the version of NSS there is installed and it's a version which should have the fix applied.

I also found posts suggesting the use of vfs_cache_pressure however increasing it does not seem to stop the usage from rising to extremely high values.

I would like to know what is a normal amount of memory of identries for a small disk < 50GB? And how can I find the source and how to fix this?

Related images:
Screenshot of slabtop: here
Graph of slab reclaimable and cached memory: here


# sysctl -n vm.vfs_cache_pressure

(Used to be 100, I increased it by x100 yet memory still increases the same amount)

# find / -type d -size +10M -ls

(no output)

As for cronjobs, besides the daily log rotation there is one script which makes a few tcp connections to obtain data and stores this in a database (Raw sockets, no curl or anything). Beside that cronjob there are 2 backup cronjobs which run once a week. The only thing which should be able to cause I/O is the apache2 webserver with SMF installed. Personally I suspect it might be mod_rewrite checking if files exist or something.

Full kernel version:

Linux #1 SMP Tue Mar 18 14:48:24 CET 2014 x86_64 x86_64 x86_64 GNU/Linux

Installed software: pastebin
Output of ps aux: pastebin

# strace -fc -e trace=access curl 'https://www.google.com' > /dev/null
Process 7342 attached
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   259  100   259    0     0    903      0 --:--:-- --:--:-- --:--:--   905
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.000048           0      7877      7872 access
------ ----------- ----------- --------- --------- ----------------
100.00    0.000048                  7877      7872 total
  • 1
    based on data from slabtop I'd say you have something constantly crawling the file system and filling up that dentry cache. Do you have some large directories on system, please add to question output of find / -type d -size +10M -ls Nov 20 '14 at 14:41
  • Thanks for the replies, and yes it seemed very related which is why I mentioned I already checked the NSS version. The nss version installed is 3.16.2-7.el7_0. The kernel installed is the default centos 7 kernel from OVH (3.10.23-xxxx-std-ipv6-64)
    – user254948
    Nov 20 '14 at 14:53
  • can you run following and update question please with output; strace -fc -e trace=access curl 'https://www.google.com' > /dev/null Nov 20 '14 at 15:56

I always use top to look for biggest VSIZE/VSS or RSS processes,

Then I go to /proc/<.PID>/ subdirectory.

And examine the smaps file for biggest offending object file, socket, and often times, library.

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