Merge bitcoin/bitcoin#24707: doc: Speed up functional test runs using ramdisk

1764849 doc: Speed up functional test runs using ramdisk (willcl-ark)

Pull request description:

  Using a ramdisk for the functional tests can give noticable speedups for developers and reviewers.

  Local testing with an 8GB ramdisk saw a full test run using `test/functional/test_runner.py --jobs=100 --cachedir=/mnt/tmp/cache --tmpdir=/mnt/tmp` reduced from ~280 seconds to ~99 seconds.

  Possible bikeshedding opportunity to be had over whether this might best fit into `doc/productivity.md`, but IMO more people will likely see it (and it will therefore be more useful) if it is here.

  It seems best to select `tmpfs` over `ramfs` as `ramfs` can grow dynamically (good) but cannot be limited in size and might cause the system to hang if you run out of ram (bad), whereas `tmpfs` is size-limited and will overflow into swap.

ACKs for top commit:
  josibake:
    ACK bitcoin/bitcoin@1764849
  jamesob:
    ACK bitcoin/bitcoin@1764849

Tree-SHA512: b8e0846d4558a7a33fbb7cd190e30c36182db36095e1c1feae8c10a12042cff9d97739964bd9211d8564231dc99b4be5eed806d12a1d11dfa908157d7f26cc67

Author: MarcoFalke

test/README.md

@@ -107,6 +107,34 @@ how many jobs to run, append `--jobs=n`
 The individual tests and the test_runner harness have many command-line
 options. Run `test/functional/test_runner.py -h` to see them all.
 
+#### Speed up test runs with a ramdisk
+
+If you have available RAM on your system you can create a ramdisk to use as the `cache` and `tmp` directories for the functional tests in order to speed them up.
+Speed-up amount varies on each system (and according to your ram speed and other variables), but a 2-3x speed-up is not uncommon.
+
+To create a 4GB ramdisk on Linux at `/mnt/tmp/`:
+
+```bash
+sudo mkdir -p /mnt/tmp
+sudo mount -t tmpfs -o size=4g tmpfs /mnt/tmp/
+```
+
+Configure the size of the ramdisk using the `size=` option.
+The size of the ramdisk needed is relative to the number of concurrent jobs the test suite runs.
+For example running the test suite with `--jobs=100` might need a 4GB ramdisk, but running with `--jobs=32` will only need a 2.5GB ramdisk.
+
+To use, run the test suite specifying the ramdisk as the `cachedir` and `tmpdir`:
+
+```bash
+test/functional/test_runner.py --cachedir=/mnt/tmp/cache --tmpdir=/mnt/tmp
+```
+
+Once finished with the tests and the disk, and to free the ram, simply unmount the disk:
+
+```bash
+sudo umount /mnt/tmp
+```
+
 #### Troubleshooting and debugging test failures
 
 ##### Resource contention