One thread on Ubuntu reports n/a after about 2 days
The miner starts up and begins working just fine on my Vega56 rig. After running for roughly 2 days, one thread will show n/a. I generally have to press and hold the power button to actually get my computer to shutdown at this point. Killing the process and running `sudo poweroff` isn't even sufficient as it seems that something is badly hung, I guess in the AMD driver?

I just rebooted and restarted a few minutes ago. It generally starts out fine, like so. In another day or two if I check again, thread 10 will report as n/a.

```
$ sudo ./xmrig-amd
 * ABOUT        XMRig-AMD/2.8.4-dev gcc/7.3.0
 * LIBS         libuv/1.18.0 OpenCL/2.0 OpenSSL/1.1.0g
 * CPU          Intel(R) Celeron(R) CPU G3900 @ 2.80GHz x64 AES
 * ALGO         cryptonight, donate=1%
 * POOL #1      gulf.moneroocean.stream:10001 variant auto
 * COMMANDS     hashrate, pause, resume
[2018-10-29 20:48:56] compiling code and initializing GPUs. This will take a while...
[2018-10-29 20:48:56] #0, GPU #0 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:56]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:56] #1, GPU #0 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:56]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:56] #2, GPU #0 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:56]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:56] #3, GPU #0 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:56]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:56] #4, GPU #1 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #5, GPU #1 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #6, GPU #1 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #7, GPU #1 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #8, GPU #2 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #9, GPU #2 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #10, GPU #2 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:48:57] #11, GPU #2 Radeon RX Vega, intensity: 896 (8/256), unroll: 8, cu: 56
[2018-10-29 20:48:57]            CACHE: ./.cache/XILUEUNHXT5232YS74U6XXBSRKDYFSWMJINXM2A6EEFVU7VUGOCQ.bin
[2018-10-29 20:49:03] use pool gulf.moneroocean.stream:10001  142.44.240.132
[2018-10-29 20:49:03] new job from gulf.moneroocean.stream:10001 diff 1000 algo cn/2
<snip>
[2018-10-29 20:55:00] speed 10s/60s/15m 4999.3 4957.2 n/a H/s max 5759.1 H/s
| THREAD | GPU | 10s H/s | 60s H/s | 15m H/s |
|      0 |   0 |   421.0 |   420.3 |     n/a |
|      1 |   0 |   421.0 |   420.2 |     n/a |
|      2 |   0 |   420.9 |   420.3 |     n/a |
|      3 |   0 |   420.9 |   420.2 |     n/a |
|      4 |   1 |   401.7 |   400.5 |     n/a |
|      5 |   1 |   401.3 |   400.2 |     n/a |
|      6 |   1 |   401.7 |   400.5 |     n/a |
|      7 |   1 |   401.5 |   400.2 |     n/a |
|      8 |   2 |   420.9 |   420.3 |     n/a |
|      9 |   2 |   420.9 |   420.3 |     n/a |
|     10 |   2 |   421.0 |   420.2 |     n/a |
|     11 |   2 |   420.9 |   420.3 |     n/a |
```


After running for about 2 days, thread 10 (and it is _always_ thread 10) will report as n/a for all time intervals. The cache file was generated using the 18.10 driver, and I upgraded to 18.30. A few days ago, I attempted to upgrade to 18.40 to see if that made any difference. Unfortunately, the same problem just occurred and I had to power cycle the computer, and restart xmrig-amd. Thoughts on how to troubleshoot this? Note that I did compile with flags `-DSTRICT_CACHE=OFF -DWITH_HTTPD=OFF`