Merge bitcoin/bitcoin#27124: docs: add ramdisk guide for running tests on OSX

2f84ad7b9e62dd710940c2f265b65973b94864d7 docs: add ramdisk guide for running tests on OSX (Matthew Zipkin)

Pull request description:

  Using a ramdisk on OSX sped up the test suite by about 5x (using default `jobs=4`) on my M1 macbook pro running macOS Monterey 12.3.1. This PR adds the relevant OSX commands following the Linux directions.

  Default:
  ```
  8204 s (accumulated)
  Runtime: 2104 s
  ```

  following commands from the PR:
  ```
  1606 s (accumulated)
  Runtime: 421 s
  ```

  ramdisk + `jobs=32`:
  ```
  2090 s (accumulated)
  Runtime: 85 s
  ```

ACKs for top commit:
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Tree-SHA512: 37a9903c8ac2cbfaa91e7e73fc96ef65042ff4b15763d452af7b8615255adf03429ad01cf85265a99dd569290c1d69c05a393d616868c05c190b60b053820786

1 files changed, 32 insertions, 9 deletions

diff --git a/test/README.md b/test/README.md
index fdbb91832a..0eddb72e1f 100644
--- a/test/README.md
+++ b/test/README.md
@@ -109,34 +109,57 @@ 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
+#### Speed up test runs with a RAM disk
 
-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.
+If you have available RAM on your system you can create a RAM disk 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/`:
+**Linux**
+
+To create a 4 GiB RAM disk 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.
+Configure the size of the RAM disk using the `size=` option.
+The size of the RAM disk 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 4 GiB RAM disk, but running with `--jobs=32` will only need a 2.5 GiB RAM disk.
 
-To use, run the test suite specifying the ramdisk as the `cachedir` and `tmpdir`:
+To use, run the test suite specifying the RAM disk 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:
+Once finished with the tests and the disk, and to free the RAM, simply unmount the disk:
 
 ```bash
 sudo umount /mnt/tmp
 ```
 
+**macOS**
+
+To create a 4 GiB RAM disk named "ramdisk" at `/Volumes/ramdisk/`:
+
+```bash
+diskutil erasevolume HFS+ ramdisk $(hdiutil attach -nomount ram://8388608)
+```
+
+Configure the RAM disk size, expressed as the number of blocks, at the end of the command
+(`4096 MiB * 2048 blocks/MiB = 8388608 blocks` for 4 GiB). To run the tests using the RAM disk:
+
+```bash
+test/functional/test_runner.py --cachedir=/Volumes/ramdisk/cache --tmpdir=/Volumes/ramdisk/tmp
+```
+
+To unmount:
+
+```bash
+umount /Volumes/ramdisk
+```
+
 #### Troubleshooting and debugging test failures
 
 ##### Resource contention