Document the continuous profiling of computes

docs/continuous-profiling.md

@@ -0,0 +1,58 @@
+# Continuous Crofiling (Compute)
+
+The continuous profiling of the compute node is performed by `perf` or `bcc-tools`, the latter is preferred.
+
+The executables profiled are all the postgres-related ones only, excluding the actual compute code (Rust). This can be done as well but
+was not the main goal.
+
+## Tools
+
+The aforementioned tools are available within the same Docker image as
+the compute node itself, but the corresponding dependencies linux the
+linux kernel headers and the linux kernel itself are not and can't be
+for obvious reasons. To solve that, as we run the compute nodes as a
+virtual machine (qemu), we need to deliver these dependencies to it.
+This is done by the `autoscaling` part, which builds and deploys the
+kernel headers, needed modules, and the `perf` binary into an ext4-fs
+disk image, which is later attached to the VM and is symlinked to be
+made available for the compute node.
+
+## Output
+
+The output of the profiling is always a binary file in the same format
+of `pprof`. It can, however, be archived by `gzip` additionally, if the
+corresponding argument is provided in the JSON request.
+
+## REST API
+
+### Test profiling
+
+One can test the profiling after connecting to the VM and running:
+
+```sh
+curl -X POST -H "Content-Type: application/json" http://localhost:3080/profile/cpu -d '{"profiler": {"BccProfile": null}, "sampling_frequency": 99, "timeout_seconds": 5, "archive": false}' -v --output profile.pb
+```
+
+This uses the `Bcc` profiler and does not archive the output. The
+profiling data will be saved into the `profile.pb` file locally.
+
+**Only one profiling session can be run at a time.**
+
+To check the profiling status (to see whether it is already running or
+not), one can perform the `GET` request:
+
+```sh
+curl http://localhost:3080/profile/cpu -v
+```
+
+The profiling can be stopped by performing the `DELETE` request:
+
+```sh
+curl -X DELETE http://localhost:3080/profile/cpu -v
+```
+
+## Supported profiling data
+
+For now, only the CPU profiling is done and ther is no heap profiling.
+Also, only the postgres-related executables are tracked, the compute
+(Rust) part itself **is not tracked**.