document how to analyze performance / memory usage issues

CONTRIBUTING.md

@@ -88,10 +88,40 @@ Then use the `go` tool to build restic:
     $ ./restic version
     restic 0.14.0-dev (compiled manually) compiled with go1.19 on linux/amd64
 
+To create a debug build use:
+
+    $ go build -tags debug ./cmd/restic
+
 You can run all tests with the following command:
 
     $ go test ./...
 
+
+Performance and Memory Usage Issues
+===================================
+
+Debug builds of restic support the `--block-profile`, `--cpu-profile`,
+`--mem-profile`, and `--trace-profile` options which collect performance data
+that later on can be analyzed using the go tools:
+
+    $ restic --cpu-profile . [...]
+    $ go tool pprof -http localhost:12345 cpu.pprof
+
+To analyze a trace profile use `go tool trace -http=localhost:12345 trace.out`.
+
+As the memory usage of restic changes over time, it may be useful to capture a
+snapshot of the current heap. This is possible using then `--listen-profile`
+option. Then while restic runs you can query and afterwards analyze the heap statistics.
+
+    $ restic --listen-profile localhost:12345 [...]
+    $ curl http://localhost:12345/debug/pprof/heap -o heap.pprof
+    $ go tool pprof -http localhost:12345 heap.pprof
+
+Further useful tools are setting the environment variable `GODEBUG=gctrace=1`,
+which provides information about garbage collector runs. For a graphical variant
+combine this with gcvis.
+
+
 Providing Patches
 =================