Update fleet-scaling.md

Documentation/fleet-scaling.md

@@ -1,23 +1,37 @@
 # fleet and scaling
 
-As fleet uses etcd for cluster wide coordination, making it scale requires
-minimizing the load it puts on etcd. This is true for reads, writes, and
-watches.
+As fleet currently uses etcd for cluster wide coordination, making it scale 
+requires minimizing the load it puts on etcd. This is true for reads, writes,
+and watches.
 
 ## Known issues
 
-Currently when fleet schedules a job *all* `fleetd`s are woken up (via a watch)
+- Currently when fleet schedules a job *all* `fleetd`s are woken up (via a watch)
 and then do a recursive GET on the Unit file in etcd to figure out if it should
 schedule a job. This is a very expensive operation.
 
+- With a large number of units (~500+), the `fleetd` agent exhibits significant 
+CPU usage from parsing all D-Bus messages when interacting with systemd.
+
+- With a large number of units, `fleetd` exhibits significant CPU usage when 
+  - a) parsing the JSON-encoded representations (stored in etcd)
+  - b) parsing the unit file itself (using go-systemd)
+
+- The agent deals very poorly with inconsistent read/write latencies with etcdal
+  (*what is the actual behaviour?*) 
+
 ## Improvements
 
-To make fleet scale further in the future we should consider rearchitecting
+To make fleet scale further in the future we could consider rearchitecting
 fleet to provide per-machine schedules (so that only those `fleetd`s are woken
 up that actually have work for them). This is akin to the [Thundering herd
 problem](https://en.wikipedia.org/wiki/Thundering_herd_problem), but in a
 distributed fashion. Once such a change is in we can also drop the periodic
-wakeups (agent TTLs) that cause fleet wide wake ups on a regular clock.
+wakeups (agent TTLs) that cause fleet wide wake-ups on a regular clock.
+
+Ultimately, fleet should move away from using etcd as an RPC mechanism. 
+Instead, it should use etcd only for leader election and then perform direct
+RPCs between the engine and agent.
 
 ## Quick wins
 
@@ -30,12 +44,14 @@ wins:
     The downside of this change is that fleet's responsiveness is lower.
     Proposal: https://github.com/coreos/fleet/pull/1264
 
+## Implemented quick wins
+
 * Disallowing (some) nodes to partake in the fleet leadership election. Again
     this is an expensive operation. The fewer nodes that are engaged in this
     election, the better. Possible downside is that if there isn't a leader at
     all, the cluster is inoperable. However the (usually) 5 machines running
-    etcd are also a single point of failure. See the `--disable-engine` flag.
+    etcd are also a single point of failure. *See the `--disable-engine` flag.*
 
 * Making some defaults exported and allow them to be overridden. For instance
-    fleet's tokenLimit controls how many Units are listed per "page". See the
-    `--token-limit` flag.
+    fleet's tokenLimit controls how many Units are listed per "page". *See the
+    `--token-limit` flag.*