Change the default low-watermark of the cache from 50% to 80%

Pruning half the cache when hitting the cache quota seems excessive as a
default value, resulting in a lower cache hit rate.

doc/source/using_config.rst

@@ -130,8 +130,8 @@ toplevel of your configuration file, like so:
      # Keep 5% of disk space available
      reserved-disk-space: 5%
 
-     # Retain 50% of the cache on cleanup
-     low-watermark: 50%
+     # Retain 80% of the cache on cleanup
+     low-watermark: 80%
 
      # Avoid pulling large amounts of data we don't need locally
      pull-buildtrees: False
@@ -205,7 +205,7 @@ Attributes
 
   ``low-watermark`` is specified as a percentage of the effective cache quota
   as configured by ``quota`` and/or ``reserved-disk-space``. The default is
-  ``50%``, which means that when cleanup is triggered, 50% of the cache will
+  ``80%``, which means that when cleanup is triggered, 20% of the cache will
   be pruned by removing CAS objects that haven't been used recently.
 
 * ``pull-buildtrees``

src/buildstream/data/userconfig.yaml

@@ -41,8 +41,8 @@ cache:
   # Keep 5% of disk space available
   reserved-disk-space: 5%
 
-  # Retain 50% of the cache on cleanup
-  low-watermark: 50%
+  # Retain 80% of the cache on cleanup
+  low-watermark: 80%
 
   # Whether to pull build trees when downloading element artifacts
   pull-buildtrees: False

tests/artifactcache/expiry.py

@@ -136,7 +136,7 @@ def test_expiry_order(cli, datafiles):
     element_path = "elements"
     checkout = os.path.join(project, "workspace")
 
-    cli.configure({"cache": {"quota": 9000000}})
+    cli.configure({"cache": {"quota": 9000000, "low-watermark": "50%"}})
 
     # Create an artifact
     create_element_size("dep.bst", project, element_path, [], 2000000)