[www@server1 tmp]$ ruby test_gc.rb 
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux]
2018-12-11 03:15:38 +0000
Memory: Process RSS/System Total Memory/System Free Memory
0m0s 22028	Memory:9.48 MB/3.85 GB/1.54 GB, objspace_mem:4.08 MB
0m3s 22028	Memory:9.49 MB/3.85 GB/1.36 GB, objspace_mem:3.39 MB
0m6s 22028	Memory:9.49 MB/3.85 GB/1.39 GB, objspace_mem:3.39 MB
0m9s 22028	Memory:9.49 MB/3.85 GB/1.15 GB, objspace_mem:3.39 MB
read content from file 214189538[204.27 MB]
0m12s 22028	Memory:213.83 MB/3.85 GB/813.78 MB, objspace_mem:207.65 MB
0m15s 22028	Memory:213.83 MB/3.85 GB/610.74 MB, objspace_mem:207.65 MB
0m18s 22028	Memory:213.83 MB/3.85 GB/578.07 MB, objspace_mem:207.65 MB
0m21s 22028	Memory:213.83 MB/3.85 GB/683.48 MB, objspace_mem:207.65 MB
0m24s 22028	Memory:213.83 MB/3.85 GB/784.64 MB, objspace_mem:207.65 MB
0m27s 22028	Memory:213.83 MB/3.85 GB/784.51 MB, objspace_mem:207.65 MB
0m30s 22028	Memory:213.83 MB/3.85 GB/784.13 MB, objspace_mem:207.65 MB
0m33s 22028	Memory:213.83 MB/3.85 GB/784.61 MB, objspace_mem:207.65 MB
0m36s 22028	Memory:213.83 MB/3.85 GB/784.4 MB, objspace_mem:207.65 MB
0m39s 22028	Memory:213.83 MB/3.85 GB/784.4 MB, objspace_mem:207.65 MB
parse json string to object
0m58s 22028	Memory:1.36 GB/3.85 GB/36.2 MB, objspace_mem:1.23 GB
1m2s 22028	Memory:1.36 GB/3.85 GB/38.11 MB, objspace_mem:1.23 GB
1m7s 22028	Memory:1.36 GB/3.85 GB/144.34 MB, objspace_mem:1.23 GB
1m11s 22028	Memory:1.36 GB/3.85 GB/36.94 MB, objspace_mem:1.23 GB
1m16s 22028	Memory:1.36 GB/3.85 GB/35.85 MB, objspace_mem:1.23 GB
1m21s 22028	Memory:1.36 GB/3.85 GB/35.29 MB, objspace_mem:1.23 GB
1m26s 22028	Memory:1.36 GB/3.85 GB/35.29 MB, objspace_mem:1.23 GB
set string&object to nil, and sleep 30 min to check the memory usage.
1m30s 22028	Memory:1.14 GB/3.85 GB/369.66 MB, objspace_mem:3.39 MB
1m33s 22028	Memory:1.14 GB/3.85 GB/368.24 MB, objspace_mem:3.39 MB
1m36s 22028	Memory:1.14 GB/3.85 GB/368.01 MB, objspace_mem:3.39 MB
1m39s 22028	Memory:1.14 GB/3.85 GB/365.01 MB, objspace_mem:3.39 MB
1m42s 22028	Memory:1.14 GB/3.85 GB/271.18 MB, objspace_mem:3.39 MB
1m45s 22028	Memory:1.04 GB/3.85 GB/141.49 MB, objspace_mem:3.39 MB
1m48s 22028	Memory:961.32 MB/3.85 GB/118.21 MB, objspace_mem:3.39 MB
1m52s 22028	Memory:960.1 MB/3.85 GB/824.84 MB, objspace_mem:3.39 MB
1m55s 22028	Memory:960.1 MB/3.85 GB/641.77 MB, objspace_mem:3.39 MB
1m58s 22028	Memory:960.1 MB/3.85 GB/452.16 MB, objspace_mem:3.39 MB
2m1s 22028	Memory:960.1 MB/3.85 GB/358.32 MB, objspace_mem:3.39 MB
2m4s 22028	Memory:960.1 MB/3.85 GB/358.2 MB, objspace_mem:3.39 MB
2m7s 22028	Memory:960.1 MB/3.85 GB/358.12 MB, objspace_mem:3.39 MB
2m10s 22028	Memory:960.1 MB/3.85 GB/357.85 MB, objspace_mem:3.39 MB
2m13s 22028	Memory:960.1 MB/3.85 GB/340.9 MB, objspace_mem:3.39 MB
2m16s 22028	Memory:960.1 MB/3.85 GB/341.32 MB, objspace_mem:3.39 MB
2m19s 22028	Memory:960.1 MB/3.85 GB/341.06 MB, objspace_mem:3.39 MB
2m22s 22028	Memory:960.1 MB/3.85 GB/341.44 MB, objspace_mem:3.39 MB
2m25s 22028	Memory:960.1 MB/3.85 GB/340.84 MB, objspace_mem:3.39 MB
2m28s 22028	Memory:960.1 MB/3.85 GB/340.86 MB, objspace_mem:3.39 MB
2m31s 22028	Memory:960.1 MB/3.85 GB/1.27 GB, objspace_mem:3.39 MB
^CTraceback (most recent call last):
	1: from test_gc.rb:83:in `<main>'
test_gc.rb:83:in `sleep': Interrupt

[www@server1 tmp]$