rate-limit: Adjust token costs of RPC calls

Rather than have the same token costs for all calls, we want to
rate limit clients based on how expensive the functions they are
calling are.

I measured the average runtime of most xapi calls, which are stored
in a hashtable and are used as the token cost for each call. This does
not account for any overheads, and will need to be adjusted - it is
merely a starting point. We also use the median as a default.

Signed-off-by: Christian Pardillo Laursen <[email protected]>

Author: cplaursen

ocaml/xapi/server_helpers.ml

@@ -188,6 +188,7 @@ let do_dispatch ?session_id ?forward_op ?self:_ supports_async called_fn_name
           Rate_limit.Bucket_table.peek Xapi_rate_limit.bucket_table ~user_agent
       )
     in
+    let token_cost = Xapi_rate_limit.get_token_cost called_fn_name in
     let handle_request () =
       match sync_ty with
       | `Sync ->
@@ -205,19 +206,19 @@ let do_dispatch ?session_id ?forward_op ?self:_ supports_async called_fn_name
       match peek_result with
       | Some tokens -> (
           D.debug
-            "Bucket table: Expecting to consume 1 token from user_agent %s \
-             with available tokens %f"
-            user_agent tokens ;
+            "Bucket table: Expecting to consume %f tokens from user_agent %s \
+             with available tokens %f in function %s"
+            token_cost user_agent tokens __FUNCTION__ ;
           match sync_ty with
           | `Sync ->
               Rate_limit.Bucket_table.submit_sync Xapi_rate_limit.bucket_table
-                ~user_agent ~callback:sync 1.
+                ~user_agent ~callback:sync token_cost
           | `Async ->
               let need_complete = not (Context.forwarded_task __context) in
               Rate_limit.Bucket_table.submit Xapi_rate_limit.bucket_table
                 ~user_agent
                 ~callback:(fun () -> async ~need_complete)
-                1. ;
+                token_cost ;
               Rpc.success (API.rpc_of_ref_task (Context.get_task_id __context))
           | `InternalAsync ->
               async ~need_complete:true ;

ocaml/xapi/xapi_http.ml

@@ -361,7 +361,7 @@ let add_handler (name, handler) =
       | Some user_agent ->
           debug "Rate limiting handler %s with user_agent %s" name user_agent ;
           Rate_limit.Bucket_table.submit Xapi_rate_limit.bucket_table
-            ~user_agent ~callback:handler 1.0
+            ~user_agent ~callback:handler Xapi_rate_limit.median_token_cost
   in
   let h req ic () =
     let client =