What to Do if Slab Cache Grows and Negatively Impacts the Server Performance

Say, you have a large partition (1Tb or higher), for example,/home, which is almost full. This means that there are a lot of users' files on it. When some software, like backup, scans all these files, the Slab cache starts to grow. Here is an example:

# cat /proc/meminfo
MemTotal:       131822716 kB
MemFree:         8396188 kB
MemAvailable:   125324428 kB
Buffers:           18040 kB
Cached:            94444 kB
SwapCached:       741444 kB
Active:           873220 kB
Inactive:         972620 kB
Active(anon):     818524 kB
Inactive(anon):   925616 kB
Active(file):      54696 kB
Inactive(file):    47004 kB
Unevictable:           0 kB
Mlocked:               0 kB
SwapTotal:      31999996 kB
SwapFree:       20412420 kB
Dirty:                 0 kB
Writeback:             0 kB
AnonPages:       1228792 kB
Mapped:            40932 kB
Shmem:              8124 kB
Slab:           120366508 kB
SReclaimable:   117102628 kB
SUnreclaim:      3263880 kB 

As you see Slab is over 100 Gb!

This can negatively impact the overall performance of your server, especially when some heavy IO software is running.

There are a few kernel settings to control the Slab cache size. They can be set with # sysctl -w option=value. To make custom kernel settings persistent upon reboot, add them to /etc/sysctl.conf

First, we recommend setting the vm.vfs_cache_min_ratio to 0.

Important notice: setting vm.vfs_cache_min_ratio to 0 for the kernel version prior to 3.10.0-614.10.2.lve1.4.46 will crash it. So, you can safely set vm.vfs_cache_min_ratio to 0 if your server is running the kernel 3.10.0-614.10.2.lve1.4.46 or higher.

vm.vfs_cache_min_ratio parameter controls the minimum amount (in percentage) of dentry\inode cache which can not be reclaimed. The default value is 2, which is rather small, but with a lot of cgroups used in the system, this can turn out to be a very big value.

If you see this error when trying to apply the parameter

"sysctl: cannot stat /proc/sys/vm/vfs_cache_min_ratio: No such file or directory"

note that /proc/sys/vm/vfs_cache_min_ratio is only available on CloudLinux 6, CloudLinux 6 Hybrid, and CloudLinux 7. It is not available on CloudLinux 7 Hybrid and CloudLinux 8.

The second parameter is vm.vfs_cache_pressure, the default value is 100. Basically, this parameter controls how aggressively the kernel will try to shrink the dentry/inode cache, so setting it to a bigger value could help to reduce the Slab cache size. The value can be set to 500 or even 1000.