[www@server1 tmp]$ ruby test_gc.rb 
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux]
2018-12-11 03:16:39 +0000
Memory: Process RSS/System Total Memory/System Free Memory
0m0s 22268	Memory:9.5 MB/3.85 GB/34.85 MB, objspace_mem:4.08 MB
0m3s 22268	Memory:9.51 MB/3.85 GB/144.21 MB, objspace_mem:3.39 MB
0m6s 22268	Memory:9.51 MB/3.85 GB/245.18 MB, objspace_mem:3.39 MB
0m9s 22268	Memory:9.51 MB/3.85 GB/245.08 MB, objspace_mem:3.39 MB
read content from file 214189538[204.27 MB]
0m12s 22268	Memory:213.85 MB/3.85 GB/35.93 MB, objspace_mem:207.65 MB
0m15s 22268	Memory:213.85 MB/3.85 GB/35.57 MB, objspace_mem:207.65 MB
0m18s 22268	Memory:213.85 MB/3.85 GB/35.55 MB, objspace_mem:207.65 MB
0m21s 22268	Memory:213.85 MB/3.85 GB/35.57 MB, objspace_mem:207.65 MB
0m24s 22268	Memory:213.85 MB/3.85 GB/35.0 MB, objspace_mem:207.65 MB
0m28s 22268	Memory:209.48 MB/3.85 GB/35.2 MB, objspace_mem:207.65 MB
0m31s 22268	Memory:208.99 MB/3.85 GB/370.68 MB, objspace_mem:207.65 MB
0m34s 22268	Memory:211.51 MB/3.85 GB/368.02 MB, objspace_mem:207.65 MB
0m37s 22268	Memory:211.51 MB/3.85 GB/365.3 MB, objspace_mem:207.65 MB
0m40s 22268	Memory:211.51 MB/3.85 GB/365.14 MB, objspace_mem:207.65 MB
parse json string to object
0m59s 22268	Memory:1.36 GB/3.85 GB/358.57 MB, objspace_mem:1.23 GB
1m4s 22268	Memory:1.36 GB/3.85 GB/358.2 MB, objspace_mem:1.23 GB
1m9s 22268	Memory:1.36 GB/3.85 GB/358.27 MB, objspace_mem:1.23 GB
1m14s 22268	Memory:1.36 GB/3.85 GB/341.19 MB, objspace_mem:1.23 GB
1m18s 22268	Memory:1.36 GB/3.85 GB/341.66 MB, objspace_mem:1.23 GB
1m23s 22268	Memory:1.36 GB/3.85 GB/340.98 MB, objspace_mem:1.23 GB
1m28s 22268	Memory:1.36 GB/3.85 GB/340.71 MB, objspace_mem:1.23 GB
set string&object to nil, and sleep 30 min to check the memory usage.
1m32s 22268	Memory:1.14 GB/3.85 GB/2.43 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]$