docs: document the resource requirements (#585)

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Related to #584

docs/deployment/resource-requirements.md

@@ -0,0 +1,103 @@
+---
+title: Resource requirements
+---
+
+Usual Packit Service deployment consists of the following services with these
+resource requirements.
+
+## CPU requirements
+
+| Deployment       |  Requested (always assigned) |  Limit |
+| ---------------- | ---------------------------: | -----: |
+| postgres         |                        `30m` |    `1` |
+| redict           |                        `10m` |  `10m` |
+| flower           |                         `5m` |  `50m` |
+| nginx            |                         `5m` |  `10m` |
+| pushgateway      |                         `5m` |  `10m` |
+| tokman           | `20m` (prod, `5m` otherwise) |  `50m` |
+| dashboard        |                         `5m` |  `50m` |
+| fedmsg           |                         `5m` |  `50m` |
+| beat             |                         `5m` |  `50m` |
+| service          |                        `10m` | `200m` |
+| worker (generic) |                       `100m` | `400m` |
+| worker (short)   |                        `80m` | `400m` |
+| worker (long)    |                       `100m` | `600m` |
+
+## Memory requirements
+
+| Deployment       |      Requested (always assigned) |                              Limit |
+| ---------------- | -------------------------------: | ---------------------------------: |
+| postgres         |  `1Gi` (prod, `256Mi` otherwise) | `1536Mi` (prod, `512Mi` otherwise) |
+| redict           |                          `128Mi` |                            `256Mi` |
+| flower           |                           `80Mi` |                            `128Mi` |
+| nginx            |                            `8Mi` |                             `32Mi` |
+| pushgateway      |                           `16Mi` |                             `32Mi` |
+| tokman           | `100Mi` (prod, `88Mi` otherwise) |  `160Mi` (prod, `128Mi` otherwise) |
+| dashboard        |                          `128Mi` |                            `256Mi` |
+| fedmsg           |                           `88Mi` |                            `128Mi` |
+| beat             |                          `160Mi` |                            `256Mi` |
+| service          |                          `320Mi` |    `1Gi` (prod, `512Mi` otherwise) |
+| worker (generic) |                          `384Mi` |                           `1024Mi` |
+| worker (short)   |                          `320Mi` |                            `640Mi` |
+| worker (long)    |                          `384Mi` |                           `1024Mi` |
+
+## Currently allowed requirements / limits
+
+| Resource | Allowed to request | Limit |
+| -------- | -----------------: | ----: |
+| CPU      |                `3` |  `12` |
+| Memory   |              `6Gi` | `8Gi` |
+
+## Total for production
+
+| Deployment       | Memory request | Memory limit | CPU request |   CPU limit |
+| ---------------- | -------------: | -----------: | ----------: | ----------: |
+| non-scalable[^1] |       `2052Mi` |     `3808Mi` |      `100m` |     `1480m` |
+| 2× short worker  |        `640Mi` |     `1280Mi` |      `160m` |      `800m` |
+| 2× long worker   |        `768Mi` |     `2048Mi` |      `200m` |     `1200m` |
+| **Σ**            |   **`3460Mi`** | **`7136Mi`** |  **`460m`** | **`3480m`** |
+
+## Proposed changes
+
+1. Revert to the pre-MP+ resources (they were higher for service, workers and
+   postgres; lower values were used due to a hardcoded check in the templates);
+
+   Pre-MP+ memory requirements/limits for production deployment:
+
+   | Deployment | Requested | Limit |
+   | ---------- | --------: | ----: |
+   | postgres   |     `2Gi` | `4Gi` |
+   | service    |    `320m` | `4Gi` |
+
+   With the current setup (2× short and long-running workers), we would need
+
+   | Resource |  Request |     Limit |
+   | -------- | -------: | --------: |
+   | CPU      |   `460m` |   `3480m` |
+   | Memory   | `4484Mi` | `12768Mi` |
+
+   Requesting the memory quotas to be multiplied by 3 results in having ~`11Gi`
+   memory left which should be enough to scale up for few more workers if
+   needed. This setup would also allow scaling up to 8 workers per each queue.
+
+1. Request adjustments of the quotas such that we can have some buffer (database
+   migrations, higher load on service, etc.), but also could **permanently**
+   scale up the workers if we find service to be more reliable that way
+
+   - Based on the calculations above, 2× the current quotas on memory would be
+     sufficient, but if we were to scale the workers up too (and account for
+     possible adjustments, e.g., Redict) we should probably go for 3×
+
+1. Migrate tokman to different toolchain, it's a small self-contained app, so it
+   is easy to migrate to either Rust or Go that should leave smaller footprint.
+
+   - Opened an issue for testing out running without Tokman deployment
+     https://github.com/packit/tokman/issues/72
+
+   - Opened an issue for migrating in case we need the tokman
+     To be opened, if the previous issue “fails” (i.e. tokman is still needed,
+     or dropping affects the amount of requests to GitHub in a negative way)
+
+[^1]:
+    includes non-scalable deployments, i.e., each runs just one pod, e.g.,
+    dashboard, redict, postgres, etc.