Memory leak after challenge completion

[tsujp]

Describe the issue

I'm noticing on my secure nodes that whenever a challenge is sent, of course the resident memory usage goes up; but that after the challenge is done (minutes, hours, days) the memory usage stays at around 3 GB instead of dropping back to about 650 MB.

Can you reliably reproduce the issue?

Yes, 100% of the time.

If so, please list the steps to reproduce below:

1. Launch zend
2. run htop and view memory usage before challenge
3. Run a challenge
4. run htop and view the memory usage after challenge
5. run htop any arbitrary time after (1 minute, 1 hour, 1 day) and memory usage will still be high.

Expected behaviour

It should go back down to 650 M instead of keeping 2.35 GB (3 G - 650 MB) of memory unavailable for use.

The version of Zen you were using:

2001150 and 2001151

Machine specs:

• OS name + version: BOTH ubuntu 16.04.9 and Debian 9
• RAM: 6 GB
• Disk size: 30 GB
• Disk Type (HD/SDD): SSD
• Linux kernel version (uname -a): BOTH Linux zsn12 2.6.32-042stab123.3 #1 SMP Fri May 5 12:29:05 MSK 2017 x86_64 x86_64 x86_64 GNU/Linux AND Linux zsncl01 4.9.0-6-amd64 #1 SMP Debian 4.9.82-1+deb9u3 (2018-03-02) x86_64 GNU/Linux
• Compiler version (gcc -version): BOTH gcc (Ubuntu 5.4.0-6ubuntu1~16.04.9) 5.4.0 20160609 AND gcc (Debian 6.3.0-18+deb9u1) 6.3.0 20170516