>>319 vm.vfs_cache_pressureを調整してみ。
vfs_cache_pressure
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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=100 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. Increasing vfs_cache_pressure beyond 100
causes the kernel to prefer to reclaim dentries and inodes.