Summary:
- Excel exports were awfully memory-inefficient, causing occasional docWorker
crashes. The fix is to use the "streaming writer" option of ExcelJS
https://github.com/exceljs/exceljs#streaming-xlsx-writercontents. (Empirically
on one example, max memory went down from 3G to 100M)
- It's also CPU intensive and synchronous, and can block node for tens of
seconds. The fix is to use a worker-thread. This diff uses "piscina" library
for a pool of threads.
- Additionally, adds ProcessMonitor that logs memory and cpu usage,
particularly when those change significantly.
- Also introduces request cancellation, so that a long download cancelled by
the user will cancel the work being done in the worker thread.
Test Plan:
Updated previous export tests; memory and CPU performance tested
manually by watching output of ProcessMonitor.
Difference visible in these log excerpts:
Before (total time to serve request 22 sec):
```
Telemetry processMonitor heapUsedMB=2187, heapTotalMB=2234, cpuAverage=1.13, intervalMs=17911
Telemetry processMonitor heapUsedMB=2188, heapTotalMB=2234, cpuAverage=0.66, intervalMs=5005
Telemetry processMonitor heapUsedMB=2188, heapTotalMB=2234, cpuAverage=0, intervalMs=5005
Telemetry processMonitor heapUsedMB=71, heapTotalMB=75, cpuAverage=0.13, intervalMs=5002
```
After (total time to server request 18 sec):
```
Telemetry processMonitor heapUsedMB=109, heapTotalMB=144, cpuAverage=0.5, intervalMs=5001
Telemetry processMonitor heapUsedMB=109, heapTotalMB=144, cpuAverage=1.39, intervalMs=5002
Telemetry processMonitor heapUsedMB=94, heapTotalMB=131, cpuAverage=1.13, intervalMs=5000
Telemetry processMonitor heapUsedMB=94, heapTotalMB=131, cpuAverage=1.35, intervalMs=5001
```
Note in "Before" that heapTotalMB goes up to 2GB in the first case, and "intervalMs" of 17 seconds indicates that node was unresponsive for that long. In the second case, heapTotalMB stays low, and the main thread remains responsive the whole time.
Reviewers: jarek
Reviewed By: jarek
Differential Revision: https://phab.getgrist.com/D3906
