harmony invoked OOM killer

[LeoHChen]

Describe the bug
Validators recently noticed a higher rate of signature drop on nodes in all shards.

Using dmesg | grep harmony and you can see oom killer.

ubuntu@ip-172-31-36-171:~$ dmesg | grep harmony
[1550230.874887] harmony invoked oom-killer: gfp_mask=0x100cca(GFP_HIGHUSER_MOVABLE), order=0, oom_score_adj=0
[1550230.874892] CPU: 1 PID: 42963 Comm: harmony Not tainted 5.4.0-1041-aws #43-Ubuntu
[1550230.874997] [  32209]  1000 32209  1395312   909707  7720960        0             0 harmony
[1550230.874998] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/system.slice/harmony.service,task=harmony,pid=32209,uid=1000
[1550230.875042] Out of memory: Killed process 32209 (harmony) total-vm:5581248kB, anon-rss:3638828kB, file-rss:0kB, shmem-rss:0kB, UID:1000 pgtables:7540kB oom_score_adj:0
[1550231.167912] oom_reaper: reaped process 32209 (harmony), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
[2090066.261310] [  68640]  1000 68640  1400288   908756  7692288        0             0 harmony
[2090066.261313] oom-kill:constraint=CONSTRAINT_NONE,nodemask=(null),cpuset=/,mems_allowed=0,global_oom,task_memcg=/system.slice/harmony.service,task=harmony,pid=68640,uid=1000
[2090066.261360] Out of memory: Killed process 68640 (harmony) total-vm:5601152kB, anon-rss:3635024kB, file-rss:0kB, shmem-rss:0kB, UID:1000 pgtables:7512kB oom_score_adj:0
[2090066.570406] oom_reaper: reaped process 68640 (harmony), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB