This percentage value controls the tendency of the kernel to reclaim the memory which is used for caching of directory and inode objects.
At the default value of vfs_cache_pressure=vfs_cache_pressure_denom the kernel will attempt to reclaim dentries and inodes at a “fair” rate with respect to pagecache and swapcache reclaim. Decreasing vfs_cache_pressure causes the kernel to prefer to retain dentry and inode caches. When vfs_cache_pressure=0, the kernel will never reclaim dentries and inodes due to memory pressure and this can easily lead to out-of-memory conditions. Increasing vfs_cache_pressure beyond vfs_cache_pressure_denom causes the kernel to prefer to reclaim dentries and inodes.
Increasing vfs_cache_pressure significantly beyond vfs_cache_pressure_denom may have negative performance impact. Reclaim code needs to take various locks to find freeable directory and inode objects. When vfs_cache_pressure equals (10 * vfs_cache_pressure_denom), it will look for ten times more freeable objects than there are.
Note: This setting should always be used together with vfs_cache_pressure_denom.