[www@server1 tmp]$ ruby test_gc.rb 
ruby 2.5.3p105 (2018-10-18 revision 65156) [x86_64-linux]
2018-12-11 03:14:13 +0000
Memory: Process RSS/System Total Memory/System Free Memory
0m0s 21820	Memory:9.61 MB/3.85 GB/3.28 GB, objspace_mem:4.07 MB
0m3s 21820	Memory:9.61 MB/3.85 GB/3.28 GB, objspace_mem:3.39 MB
0m6s 21820	Memory:9.61 MB/3.85 GB/3.28 GB, objspace_mem:3.39 MB
0m9s 21820	Memory:9.61 MB/3.85 GB/3.28 GB, objspace_mem:3.39 MB
read content from file 214189538[204.27 MB]
0m12s 21820	Memory:213.95 MB/3.85 GB/3.08 GB, objspace_mem:207.65 MB
0m15s 21820	Memory:213.95 MB/3.85 GB/3.08 GB, objspace_mem:207.65 MB
0m18s 21820	Memory:213.95 MB/3.85 GB/3.08 GB, objspace_mem:207.65 MB
0m21s 21820	Memory:213.95 MB/3.85 GB/3.08 GB, objspace_mem:207.65 MB
0m24s 21820	Memory:213.95 MB/3.85 GB/3.09 GB, objspace_mem:207.65 MB
0m27s 21820	Memory:213.95 MB/3.85 GB/3.09 GB, objspace_mem:207.65 MB
0m31s 21820	Memory:213.95 MB/3.85 GB/3.09 GB, objspace_mem:207.65 MB
0m34s 21820	Memory:213.95 MB/3.85 GB/3.09 GB, objspace_mem:207.65 MB
0m37s 21820	Memory:213.95 MB/3.85 GB/3.09 GB, objspace_mem:207.65 MB
0m40s 21820	Memory:213.95 MB/3.85 GB/3.09 GB, objspace_mem:207.65 MB
parse json string to object
0m58s 21820	Memory:1.36 GB/3.85 GB/1.72 GB, objspace_mem:1.23 GB
1m3s 21820	Memory:1.36 GB/3.85 GB/1.72 GB, objspace_mem:1.23 GB
1m7s 21820	Memory:1.36 GB/3.85 GB/1.71 GB, objspace_mem:1.23 GB
1m12s 21820	Memory:1.36 GB/3.85 GB/1.71 GB, objspace_mem:1.23 GB
1m17s 21820	Memory:1.36 GB/3.85 GB/1.71 GB, objspace_mem:1.23 GB
1m21s 21820	Memory:1.36 GB/3.85 GB/1.71 GB, objspace_mem:1.23 GB
1m26s 21820	Memory:1.36 GB/3.85 GB/1.35 GB, objspace_mem:1.23 GB
set string&object to nil, and sleep 30 min to check the memory usage.
1m30s 21820	Memory:1.14 GB/3.85 GB/1.42 GB, objspace_mem:3.39 MB
1m33s 21820	Memory:1.14 GB/3.85 GB/1.26 GB, objspace_mem:3.39 MB
1m36s 21820	Memory:1.14 GB/3.85 GB/864.88 MB, objspace_mem:3.39 MB
1m39s 21820	Memory:1.14 GB/3.85 GB/703.86 MB, objspace_mem:3.39 MB
1m42s 21820	Memory:1.14 GB/3.85 GB/577.93 MB, objspace_mem:3.39 MB
1m45s 21820	Memory:1.04 GB/3.85 GB/683.15 MB, objspace_mem:3.39 MB
1m48s 21820	Memory:965.34 MB/3.85 GB/784.3 MB, objspace_mem:3.39 MB
1m51s 21820	Memory:965.34 MB/3.85 GB/784.52 MB, objspace_mem:3.39 MB
1m54s 21820	Memory:965.34 MB/3.85 GB/784.11 MB, objspace_mem:3.39 MB
1m58s 21820	Memory:965.34 MB/3.85 GB/784.35 MB, objspace_mem:3.39 MB
2m1s 21820	Memory:965.34 MB/3.85 GB/784.38 MB, objspace_mem:3.39 MB
2m4s 21820	Memory:965.34 MB/3.85 GB/784.39 MB, objspace_mem:3.39 MB
2m7s 21820	Memory:965.34 MB/3.85 GB/653.25 MB, objspace_mem:3.39 MB
2m10s 21820	Memory:965.34 MB/3.85 GB/428.16 MB, objspace_mem:3.39 MB
2m13s 21820	Memory:965.34 MB/3.85 GB/165.63 MB, objspace_mem:3.39 MB
2m16s 21820	Memory:965.34 MB/3.85 GB/139.71 MB, objspace_mem:3.39 MB
2m19s 21820	Memory:965.34 MB/3.85 GB/47.18 MB, objspace_mem:3.39 MB
2m22s 21820	Memory:964.97 MB/3.85 GB/36.18 MB, objspace_mem:3.39 MB
2m25s 21820	Memory:964.97 MB/3.85 GB/35.93 MB, objspace_mem:3.39 MB
2m28s 21820	Memory:964.97 MB/3.85 GB/38.13 MB, objspace_mem:3.39 MB
2m31s 21820	Memory:964.97 MB/3.85 GB/144.21 MB, objspace_mem:3.39 MB
2m34s 21820	Memory:964.97 MB/3.85 GB/245.2 MB, objspace_mem:3.39 MB
2m37s 21820	Memory:964.86 MB/3.85 GB/36.17 MB, objspace_mem:3.39 MB
2m40s 21820	Memory:964.86 MB/3.85 GB/35.6 MB, objspace_mem:3.39 MB
2m43s 21820	Memory:964.86 MB/3.85 GB/35.53 MB, objspace_mem:3.39 MB
2m46s 21820	Memory:964.86 MB/3.85 GB/35.43 MB, objspace_mem:3.39 MB
2m49s 21820	Memory:964.86 MB/3.85 GB/35.43 MB, objspace_mem:3.39 MB
2m52s 21820	Memory:962.87 MB/3.85 GB/39.5 MB, objspace_mem:3.39 MB
2m55s 21820	Memory:871.83 MB/3.85 GB/372.46 MB, objspace_mem:3.39 MB
2m58s 21820	Memory:871.83 MB/3.85 GB/368.49 MB, objspace_mem:3.39 MB
3m1s 21820	Memory:871.83 MB/3.85 GB/368.03 MB, objspace_mem:3.39 MB
3m4s 21820	Memory:871.83 MB/3.85 GB/364.91 MB, objspace_mem:3.39 MB
3m7s 21820	Memory:871.83 MB/3.85 GB/297.21 MB, objspace_mem:3.39 MB
3m10s 21820	Memory:871.83 MB/3.85 GB/38.65 MB, objspace_mem:3.39 MB
3m13s 21820	Memory:847.86 MB/3.85 GB/36.22 MB, objspace_mem:3.39 MB
Killed
[www@server1 tmp]$