diff --git a/bin/worker.ml b/bin/worker.ml
index d47c3a21..c5b57525 100644
--- a/bin/worker.ml
+++ b/bin/worker.ml
@@ -45,7 +45,7 @@ let update_docker () =
 let update_normal () =
   Lwt.return (fun () -> Lwt.return_unit)
 
-let main ?style_renderer level ?formatter registration_path capacity name allow_push healthcheck_period prune_threshold docker_max_df_size obuilder_prune_threshold obuilder_prune_item_threshold obuilder_prune_limit state_dir obuilder additional_metrics =
+let main ?style_renderer level ?formatter registration_path capacity name allow_push healthcheck_period prune_threshold docker_max_df_size obuilder_prune_threshold obuilder_prune_limit state_dir obuilder additional_metrics =
   setup_log ?style_renderer ?formatter level;
   let update =
     if Sys.file_exists "/.dockerenv" then update_docker
@@ -57,16 +57,16 @@ let main ?style_renderer level ?formatter registration_path capacity name allow_
   Lwt_main.run begin
     let vat = Capnp_rpc_unix.client_only_vat () in
     let sr = Capnp_rpc_unix.Cap_file.load vat registration_path |> or_die in
-    Cluster_worker.run ~capacity ~name ~allow_push ~healthcheck_period ?prune_threshold ?docker_max_df_size ?obuilder_prune_threshold ?obuilder_prune_item_threshold ~obuilder_prune_limit ?obuilder ~additional_metrics ~state_dir ~update sr
+    Cluster_worker.run ~capacity ~name ~allow_push ~healthcheck_period ?prune_threshold ?docker_max_df_size ~obuilder_prune_threshold ~obuilder_prune_limit ?obuilder ~additional_metrics ~state_dir ~update sr
   end
 
 (* Command-line parsing *)
-let main ~install (style_renderer, args1) (level, args2) ((registration_path, capacity, name, allow_push, healthcheck_period, prune_threshold, docker_max_df_size, obuilder_prune_threshold, obuilder_prune_item_threshold, obuilder_prune_limit, state_dir, obuilder, additional_metrics), args3) =
+let main ~install (style_renderer, args1) (level, args2) ((registration_path, capacity, name, allow_push, healthcheck_period, prune_threshold, docker_max_df_size, obuilder_prune_threshold, obuilder_prune_limit, state_dir, obuilder, additional_metrics), args3) =
   if install then
     Ok (Winsvc_wrapper.install name "OCluster Worker" "Run a build worker" (args1 @ args2 @ args3))
   else
     Ok (Winsvc_wrapper.run name state_dir (fun ?formatter () ->
-      main ?style_renderer level ?formatter registration_path capacity name allow_push healthcheck_period prune_threshold docker_max_df_size obuilder_prune_threshold obuilder_prune_item_threshold obuilder_prune_limit state_dir obuilder additional_metrics))
+      main ?style_renderer level ?formatter registration_path capacity name allow_push healthcheck_period prune_threshold docker_max_df_size obuilder_prune_threshold obuilder_prune_limit state_dir obuilder additional_metrics))
 
 
 open Cmdliner
@@ -123,22 +123,13 @@ let docker_max_df_size =
 
 let obuilder_prune_threshold =
   Arg.value @@
-  Arg.opt Arg.(some float) None @@
+  Arg.opt Arg.float 30.0 @@
   Arg.info
     ~doc:"If using OBuilder, this threshold is used to prune the stored builds if the free space falls below this (0-100)."
     ~docv:"PERCENTAGE"
     ~docs:"OBUILDER"
     ["obuilder-prune-threshold"]
 
-let obuilder_prune_item_threshold =
-  Arg.value @@
-  Arg.opt Arg.(some int64) None @@
-  Arg.info
-    ~doc:"If using OBuilder, this threshold is used to prune the stored builds if the number of cached steps exceeds this value."
-    ~docv:"ITEMS"
-    ~docs:"OBUILDER"
-    ["obuilder-prune-item-threshold"]
-
 let obuilder_prune_limit =
   Arg.value @@
   Arg.opt Arg.int 100 @@
@@ -202,11 +193,11 @@ module Obuilder_config = struct
 end
 
 let worker_opts_t =
-  let worker_opts registration_path capacity name allow_push healthcheck_period prune_threshold docker_max_df_size obuilder_prune_threshold obuilder_prune_item_threshold obuilder_prune_limit state_dir obuilder additional_metrics =
-    (registration_path, capacity, name, allow_push, healthcheck_period, prune_threshold, docker_max_df_size, obuilder_prune_threshold, obuilder_prune_item_threshold, obuilder_prune_limit, state_dir, obuilder, additional_metrics) in
+  let worker_opts registration_path capacity name allow_push healthcheck_period prune_threshold docker_max_df_size obuilder_prune_threshold obuilder_prune_limit state_dir obuilder additional_metrics =
+    (registration_path, capacity, name, allow_push, healthcheck_period, prune_threshold, docker_max_df_size, obuilder_prune_threshold, obuilder_prune_limit, state_dir, obuilder, additional_metrics) in
   Term.(with_used_args
     (const worker_opts $ connect_addr $ capacity $ worker_name $ allow_push $ healthcheck_period
-     $ prune_threshold $ docker_max_df_size $ obuilder_prune_threshold $ obuilder_prune_item_threshold $ obuilder_prune_limit $ state_dir $ Obuilder_config.v $ additional_metrics))
+     $ prune_threshold $ docker_max_df_size $ obuilder_prune_threshold $ obuilder_prune_limit $ state_dir $ Obuilder_config.v $ additional_metrics))
 
 let cmd ~install =
   let doc = "Run a build worker" in
diff --git a/obuilder b/obuilder
index 2d0af3df..15001472 160000
--- a/obuilder
+++ b/obuilder
@@ -1 +1 @@
-Subproject commit 2d0af3df9d934c936736986e827e9d1f5b94aed9
+Subproject commit 15001472488f92a990cfb7be27fdd64d5f001162
diff --git a/test/mock_builder.ml b/test/mock_builder.ml
index cfcb36f1..49e60951 100644
--- a/test/mock_builder.ml
+++ b/test/mock_builder.ml
@@ -90,14 +90,14 @@ let update () =
   Lwt.return (fun () -> failwith "Mock restart")
 
 let run ?(capacity=1) ?(name="worker-1") ~switch t registration_service =
-  let thread = Cluster_worker.run ~switch ~capacity ~name ~build:(build t) ~healthcheck_period:600.0 ~update ~state_dir registration_service in
+  let thread = Cluster_worker.run ~switch ~capacity ~name ~build:(build t) ~update ~state_dir registration_service in
   Lwt.on_failure thread
     (fun ex -> if Lwt_switch.is_on switch then raise ex)
 
 let run_remote ~builder_switch ~network_switch ?(capacity=1) ?(name="worker-1") t registration_service =
   let thread =
     let registration_service = Mock_network.remote ~switch:network_switch registration_service in
-    Cluster_worker.run ~switch:builder_switch ~capacity ~name ~build:(build t) ~healthcheck_period:600.0 ~update ~state_dir registration_service
+    Cluster_worker.run ~switch:builder_switch ~capacity ~name ~build:(build t) ~update ~state_dir registration_service
   in
   Lwt.on_failure thread
     (fun ex ->
diff --git a/worker/cluster_worker.ml b/worker/cluster_worker.ml
index 2f4f62bc..6471dd70 100644
--- a/worker/cluster_worker.ml
+++ b/worker/cluster_worker.ml
@@ -4,49 +4,6 @@ open Capnp_rpc_lwt
 module Log_data = Log_data
 module Process = Process
 
-module Metrics = struct
-  open Prometheus
-
-  let namespace = "ocluster"
-  let subsystem = "worker"
-
-  let jobs_accepted =
-    let help = "Number of jobs accepted in total" in
-    Counter.v ~help ~namespace ~subsystem "jobs_accepted_total"
-
-  let job_time =
-    let help = "Time jobs ran for" in
-    Summary.v_label ~label_name:"result" ~help ~namespace ~subsystem "job_time_seconds"
-
-  let docker_push_time =
-    let help = "Time uploading to Docker Hub" in
-    Summary.v ~help ~namespace ~subsystem "docker_push_time_seconds"
-
-  let docker_prune_time =
-    let help = "Time spent pruning Docker cache" in
-    Summary.v ~help ~namespace ~subsystem "docker_prune_time_seconds"
-
-  let running_jobs =
-    let help = "Number of jobs currently running" in
-    Gauge.v ~help ~namespace ~subsystem "running_jobs"
-
-  let healthcheck_time =
-    let help = "Time to perform last healthcheck" in
-    Gauge.v ~help ~namespace ~subsystem "healthcheck_time_seconds"
-
-  let unhealthy =
-    let help = "Number of unhealthy workers" in
-    Gauge.v ~help ~namespace ~subsystem "unhealthy"
-
-  let cache_hits =
-    let help = "Number of OBuilder cache hits" in
-    Gauge.v ~help ~namespace ~subsystem "cache_hits"
-
-  let cache_misses =
-    let help = "Number of OBuilder cache misses" in
-    Gauge.v ~help ~namespace ~subsystem "cache_misses"
-end
-
 let buildkit_env =
   let orig = Unix.environment () |> Array.to_list in
   "DOCKER_BUILDKIT=1" :: orig |> Array.of_list
@@ -310,7 +267,7 @@ let loop ~switch ?obuilder t queue =
         check_health t ~last_healthcheck ~queue obuilder >>= fun () ->
         let outcome, set_outcome = Lwt.wait () in
         let log = Log_data.create () in
-        Log.info (fun f -> f "Requesting a new job…");
+        Log.info (fun f -> f "Requesting a new job… (%i running)" t.in_use);
         let switch = Lwt_switch.create () in
         let pop =
           Capability.with_ref (Cluster_api.Job.local ~switch ~outcome ~stream_log_data:(Log_data.stream log)) @@ fun job ->
@@ -318,7 +275,16 @@ let loop ~switch ?obuilder t queue =
         in
         t.cancel <- (fun () -> Lwt.cancel pop);
         pop >>= fun request ->
-        t.in_use <- t.in_use + 1;
+        let module R = Cluster_api.Raw.Reader.JobDescr in
+        let cache_hint = R.cache_hint_get request in
+        let weights = [
+          (Str.regexp {|.*tezos.*|}, 2);
+          (Str.regexp {|.*octez.*|}, 3);
+        ] in
+        let weight = List.fold_left (fun a (re, w) -> if Str.string_match re cache_hint 0 then w else a) 1 weights in
+        t.in_use <- t.in_use + weight;
+        Log.info (fun f -> f "Cache_hint %s" cache_hint);
+        Log.info (fun f -> f "Job weight %i" weight);
         Prometheus.Gauge.set Metrics.running_jobs (float_of_int t.in_use);
         Prometheus.Counter.inc_one Metrics.jobs_accepted;
         Lwt.async (fun () ->
@@ -333,6 +299,7 @@ let loop ~switch ?obuilder t queue =
                    (fun (outcome, metric_label) ->
                       let t1 = Unix.gettimeofday () in
                       Prometheus.Summary.observe (Metrics.job_time metric_label) (t1 -. t0);
+                      Log.info (fun f -> f "Build duration: %fs" (t1 -. t0));
                       Log_data.close log;
                       Lwt.wakeup set_outcome outcome;
                       Lwt.return_unit)
@@ -346,7 +313,7 @@ let loop ~switch ?obuilder t queue =
                       Lwt.return_unit)
               )
               (fun () ->
-                 t.in_use <- t.in_use - 1;
+                 t.in_use <- t.in_use - weight;
                  Prometheus.Gauge.set Metrics.running_jobs (float_of_int t.in_use);
                  let h, m = cache_stats obuilder in
                  Prometheus.Gauge.set Metrics.cache_hits (float_of_int h);
@@ -501,7 +468,7 @@ let self_update ~update t =
        Lwt_result.fail (`Msg (Printexc.to_string ex))
     )
 
-let run ?switch ?build ?(allow_push=[]) ~healthcheck_period ?prune_threshold ?docker_max_df_size ?obuilder_prune_threshold ?obuilder_prune_item_threshold ?obuilder_prune_limit ?obuilder ?(additional_metrics=[]) ~update ~capacity ~name ~state_dir registration_service =
+let run ?switch ?build ?(allow_push=[]) ?(healthcheck_period = 600.0) ?prune_threshold ?docker_max_df_size ?(obuilder_prune_threshold = 30.0) ?(obuilder_prune_limit = 100) ?obuilder ?(additional_metrics=[]) ~update ~capacity ~name ~state_dir registration_service =
   begin match prune_threshold, docker_max_df_size with
     | None, None -> Log.info (fun f -> f "Prune threshold not set and docker max df size is not. Will not check for low disk-space!")
     | None, Some size -> Log.info (fun f -> f "Pruning docker whenever the memory used exceeds %3.2fGB" size)
@@ -510,7 +477,7 @@ let run ?switch ?build ?(allow_push=[]) ~healthcheck_period ?prune_threshold ?do
   end;
   begin match obuilder with
     | None -> Lwt.return_none
-    | Some config -> Obuilder_build.create ?prune_threshold:obuilder_prune_threshold ?prune_item_threshold:obuilder_prune_item_threshold ?prune_limit:obuilder_prune_limit config >|= Option.some
+    | Some config -> Obuilder_build.create ~prune_threshold:obuilder_prune_threshold ~prune_limit:obuilder_prune_limit config >|= Option.some
   end >>= fun obuilder ->
   let build =
     match build with
diff --git a/worker/cluster_worker.mli b/worker/cluster_worker.mli
index b6c9167f..9e411cae 100644
--- a/worker/cluster_worker.mli
+++ b/worker/cluster_worker.mli
@@ -21,11 +21,10 @@ val run :
   ?switch:Lwt_switch.t ->
   ?build:build ->
   ?allow_push:string list ->
-  healthcheck_period:float ->
+  ?healthcheck_period:float ->
   ?prune_threshold:float ->
   ?docker_max_df_size:float ->
   ?obuilder_prune_threshold:float ->
-  ?obuilder_prune_item_threshold:int64 ->
   ?obuilder_prune_limit:int ->
   ?obuilder:Obuilder_config.t ->
   ?additional_metrics:(string * Uri.t) list ->
diff --git a/worker/dune b/worker/dune
index 7edb9d59..ae4c768a 100644
--- a/worker/dune
+++ b/worker/dune
@@ -13,4 +13,4 @@
 (library
  (name cluster_worker)
  (public_name ocluster-worker)
- (libraries ocluster-api digestif fpath logs capnp-rpc-lwt lwt.unix prometheus-app cohttp-lwt-unix obuilder extunix))
+ (libraries ocluster-api digestif fpath logs capnp-rpc-lwt lwt.unix prometheus-app cohttp-lwt-unix obuilder extunix str))
diff --git a/worker/metrics.ml b/worker/metrics.ml
new file mode 100644
index 00000000..877ccc15
--- /dev/null
+++ b/worker/metrics.ml
@@ -0,0 +1,45 @@
+open Prometheus
+
+let namespace = "ocluster"
+let subsystem = "worker"
+
+let jobs_accepted =
+  let help = "Number of jobs accepted in total" in
+  Counter.v ~help ~namespace ~subsystem "jobs_accepted_total"
+
+let job_time =
+  let help = "Time jobs ran for" in
+  Summary.v_label ~label_name:"result" ~help ~namespace ~subsystem "job_time_seconds"
+
+let docker_push_time =
+  let help = "Time uploading to Docker Hub" in
+  Summary.v ~help ~namespace ~subsystem "docker_push_time_seconds"
+
+let docker_prune_time =
+  let help = "Time spent pruning Docker cache" in
+  Summary.v ~help ~namespace ~subsystem "docker_prune_time_seconds"
+
+let running_jobs =
+  let help = "Number of jobs currently running" in
+  Gauge.v ~help ~namespace ~subsystem "running_jobs"
+
+let healthcheck_time =
+  let help = "Time to perform last healthcheck" in
+  Gauge.v ~help ~namespace ~subsystem "healthcheck_time_seconds"
+
+let unhealthy =
+  let help = "Number of unhealthy workers" in
+  Gauge.v ~help ~namespace ~subsystem "unhealthy"
+
+let cache_hits =
+  let help = "Number of OBuilder cache hits" in
+  Gauge.v ~help ~namespace ~subsystem "cache_hits"
+
+let cache_misses =
+  let help = "Number of OBuilder cache misses" in
+  Gauge.v ~help ~namespace ~subsystem "cache_misses"
+
+let obuilder_space_free =
+  let help = "OBuilder percentage of space free" in
+  Gauge.v ~help ~namespace ~subsystem "obuilder_space_free"
+
diff --git a/worker/obuilder_build.ml b/worker/obuilder_build.ml
index 9b2d35c4..31df0eaf 100644
--- a/worker/obuilder_build.ml
+++ b/worker/obuilder_build.ml
@@ -1,7 +1,5 @@
 open Lwt.Infix
 
-let prune_margin = 600.0        (* Don't prune anything used less than 10 minutes ago *)
-
 type builder = Builder : (module Obuilder.BUILDER with type t = 'a) * 'a -> builder
 
 module Config = struct
@@ -18,9 +16,6 @@ type t = {
   builder : builder;
   mutable pruning : bool;
   cond : unit Lwt_condition.t;          (* Fires when we finish pruning *)
-  prune_threshold : float option;
-  prune_item_threshold : int64 option;  (* Threshold number of items to hold in obuilder store *)
-  prune_limit : int option;             (* Number of items to prune from obuilder when threshold is reached *)
 }
 
 let ( / ) = Filename.concat
@@ -37,7 +32,7 @@ let log_to log_data tag msg =
   | `Note -> Log_data.info log_data "\027[01;2m\027[01;35m%a %s\027[0m" pp_timestamp (Unix.gettimeofday ()) msg
   | `Output -> Log_data.write log_data msg
 
-let create ?prune_threshold ?prune_item_threshold ?prune_limit config =
+let create ?(prune_threshold = 30.0) ?(prune_limit = 100) config =
   let { Config.store; sandbox_config } = config in
   store >>= fun (Obuilder.Store_spec.Store ((module Store), store)) ->
   begin match sandbox_config with
@@ -58,79 +53,36 @@ let create ?prune_threshold ?prune_item_threshold ?prune_limit config =
   | Error (`Msg m) -> Fmt.failwith "Initial OBuilder healthcheck failed: %s" m
   | Ok () ->
     Log.info (fun f -> f "OBuilder self-test passed");
+    let r =
     {
       builder = Builder ((module Builder), builder);
       pruning = false;
-      prune_threshold;
-      prune_item_threshold;
-      prune_limit;
       cond = Lwt_condition.create ();
-    }
-
-(* Prune [t] until free space rises above [prune_threshold]
-   or number of items falls below [prune_item_threshold]. *)
-let do_prune ~prune_threshold ~prune_item_threshold ~prune_limit t =
-  let Builder ((module Builder), builder) = t.builder in
-  let rec aux () =
-    let stop = Unix.gettimeofday () -. prune_margin |> Unix.gmtime in
-    Builder.prune builder ~before:stop prune_limit >>= fun n ->
-    Builder.df builder >>= fun free ->
-    let count = Builder.count builder in
-    Log.info (fun f -> f "OBuilder partition: %.0f%% free, %Li items after pruning %d items" free count n);
-    if free > prune_threshold && count < prune_item_threshold
-    then Lwt.return_unit      (* Space problem is fixed! *)
-    else if n < prune_limit then (
-      Log.warn (fun f -> f "Out of space, but nothing left to prune! (will wait and then retry)");
-      Lwt_unix.sleep 600.0 >>= aux
-    ) else (
-      (* Continue pruning *)
-      aux ()
-    )
-  in
-  aux ()
-
-(* Check the free space and/or number of items in [t]'s store.
-   If less than [t.prune_threshold] or items > [t.prune_item_threshold], spawn a prune operation (if not already running).
-   If less than half that is remaining, also wait for it to finish.
-   Returns once there is enough free space to proceed. *)
-let check_free_space t =
-  let prune_limit = Option.value t.prune_limit ~default:100 in
-  let prune_threshold = Option.value t.prune_threshold ~default:0. in
-  let prune_item_threshold = Option.value t.prune_item_threshold ~default:Int64.max_int in
-  if prune_threshold = 0. && prune_item_threshold = Int64.max_int then
-    Lwt.return_unit (* No limits have been set *)
-  else
-    let Builder ((module Builder), builder) = t.builder in
-    let rec aux () =
-      Builder.df builder >>= fun free ->
-      let count = Builder.count builder in
-      Log.info (fun f -> f "OBuilder partition: %.0f%% free, %Li items" free count);
-      (* If we're low on space, or over the threshold number of items spawn a pruning thread. *)
-      if ((prune_threshold > 0. && free < prune_threshold) ||
-          (prune_item_threshold < Int64.max_int && count > prune_item_threshold)) && not t.pruning then (
-        t.pruning <- true;
-        Lwt.async (fun () ->
-            Lwt.finalize
-              (fun () -> do_prune ~prune_threshold ~prune_item_threshold ~prune_limit t)
-              (fun () ->
-                 Lwt.pause () >|= fun () ->
-                 t.pruning <- false;
-                 Lwt_condition.broadcast t.cond ()
-              )
-          );
-      );
-      if free < prune_threshold /. 2.0 then (
-        assert (t.pruning);
-        Log.info (fun f -> f "OBuilder space very low. Waiting for prune to finish…");
-        Lwt_condition.wait t.cond >>= aux
-      ) else (
-        Lwt.return_unit
-      )
-    in
-    aux ()
+    } in
+    Lwt.async (fun () ->
+      let rec loop () =
+        Builder.df builder >>= fun free ->
+        let count = Builder.count builder in
+        Log.info (fun f -> f "OBuilder partition: %.0f%% free, %Li items" free count);
+        Prometheus.Gauge.set Metrics.obuilder_space_free free;
+        if free > prune_threshold then (
+          r.pruning <- false;
+          Lwt_condition.signal r.cond (); (* release one waiting process *)
+          Lwt_unix.sleep 30.0 >>= fun () -> loop ()
+        ) else (
+          r.pruning <- true;
+          let stop = Unix.gettimeofday () |> Unix.gmtime in
+          Builder.prune builder ~before:stop prune_limit >>= fun n ->
+          Log.info (fun f -> f "Pruned %i items" n);
+          (if n = 0 then Lwt_unix.sleep 30.0
+          else Lwt.return_unit )>>= fun () -> loop ()
+        )
+      in loop ()
+    ); r
 
 let build t ~switch ~log ~spec ~src_dir ~secrets =
-  check_free_space t >>= fun () ->
+  (if t.pruning then Lwt_condition.wait t.cond
+  else Lwt.return ()) >>= fun () ->
   let log = log_to log in
   let context = Obuilder.Context.v ~switch ~log ~src_dir ~secrets () in
   let Builder ((module Builder), builder) = t.builder in
diff --git a/worker/obuilder_build.mli b/worker/obuilder_build.mli
index 2b523b2b..60c3ebb8 100644
--- a/worker/obuilder_build.mli
+++ b/worker/obuilder_build.mli
@@ -8,7 +8,7 @@ module Config : sig
           -> Obuilder.Store_spec.store Lwt.t -> t
 end
 
-val create : ?prune_threshold:float -> ?prune_item_threshold:int64 -> ?prune_limit:int -> Config.t -> t Lwt.t
+val create : ?prune_threshold:float -> ?prune_limit:int -> Config.t -> t Lwt.t
 
 val build : t ->
   switch:Lwt_switch.t ->