Document automatic CPU/IO-concurrency

changelog/unreleased/issue-2162

@@ -0,0 +1,18 @@
+Enhancement: Adapt IO concurrency based on backend connections
+
+Many commands used hard-coded limits for the number of concurrent operations.
+This prevented speed improvements by increasing the number of connections used
+by a backend.
+
+These limits have been replaced by using the configured number of backend
+connections instead. It can be controlled using the
+`-o <backend-name>.connections=5` option. Commands will then automatically
+scale their parallelism accordingly.
+
+To limit the number of CPU cores used by restic, you can set the environment
+variable `GOMAXPROCS` accordingly. For example to use a single CPU core, use
+`GOMAXPROCS=1`.
+
+https://github.com/restic/restic/issues/2162
+https://github.com/restic/restic/issues/1467
+https://github.com/restic/restic/pull/3611

doc/047_tuning_backup_parameters.rst

@@ -30,6 +30,15 @@ to increase the number of connections. Please be aware that this increases the r
 consumption of restic and that a too high connection count *will degrade performace*.
 
 
+CPU Usage
+=========
+
+By default, restic uses all available CPU cores. You can set the environment variable
+`GOMAXPROCS` to limit the number of used CPU cores. For example to use a single CPU core,
+use `GOMAXPROCS=1`. Limiting the number of usable CPU cores, can slightly reduce the memory
+usage of restic.
+
+
 Compression
 ===========