oxenstored: don't wake to issue no conflict-credit

In the main loop, when choosing the timeout for the select function
call, we were setting it so as to wake up to issue conflict-credit to
any domains that could accept it. When xenstore is idle, this would
mean waking up every 50ms (by default) to do no work. With this
commit, we check whether any domain is below its cap, and if not then
we set the timeout for longer (the same timeout as before the
conflict-protection feature was added).

Reported-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Thomas Sanders <thomas.sanders@citrix.com>
Reviewed-by: Jonathan Davies <jonathan.davies@citrix.com>

diff --git a/tools/ocaml/xenstored/domains.ml b/tools/ocaml/xenstored/domains.ml
index 99f68c7d4bdc270b890960b71a874263ca736341..61d1e2ee0ea632e8518c36e98e5bd00fa18fbfbe 100644 (file)
--- a/tools/ocaml/xenstored/domains.ml
+++ b/tools/ocaml/xenstored/domains.ml
@@ -35,8 +35,9 @@ type domains = {
        on_first_conflict_pause: unit -> unit;
 
        (* If config is set to use individual instead of aggregate conflict-rate-limiting,
-          we use this instead of the queues. *)
-       mutable n_paused: int;
+          we use these counts instead of the queues. The second one includes the first. *)
+       mutable n_paused: int;    (* Number of domains with zero or negative credit *)
+       mutable n_penalised: int; (* Number of domains with less than maximum credit *)
 }
 
 let init eventchn on_first_conflict_pause = {
@@ -46,6 +47,7 @@ let init eventchn on_first_conflict_pause = {
        doms_with_conflict_penalty = Queue.create ();
        on_first_conflict_pause = on_first_conflict_pause;
        n_paused = 0;
+       n_penalised = 0;
 }
 let del doms id = Hashtbl.remove doms.table id
 let exist doms id = Hashtbl.mem doms.table id
@@ -53,6 +55,23 @@ let find doms id = Hashtbl.find doms.table id
 let number doms = Hashtbl.length doms.table
 let iter doms fct = Hashtbl.iter (fun _ b -> fct b) doms.table
 
+let rec is_empty_queue q =
+       Queue.is_empty q ||
+               if !(Queue.peek q) = None
+               then (
+                       ignore (Queue.pop q);
+                       is_empty_queue q
+               ) else false
+
+let all_at_max_credit doms =
+       if !Define.conflict_rate_limit_is_aggregate
+       then
+               (* Check both becuase if burst limit is 1.0 then a domain can go straight
+                * from max-credit to paused without getting into the penalty queue. *)
+               is_empty_queue doms.doms_with_conflict_penalty
+               && is_empty_queue doms.doms_conflict_paused
+       else doms.n_penalised = 0
+
 (* Functions to handle queues of domains given that the domain might be deleted while in a queue. *)
 let push dom queue =
        Queue.push (ref (Some dom)) queue
@@ -130,13 +149,16 @@ let decr_conflict_credit doms dom =
        let before = dom.Domain.conflict_credit in
        let after = max (-1.0) (before -. 1.0) in
        dom.Domain.conflict_credit <- after;
+       let newly_penalised =
+               before >= !Define.conflict_burst_limit
+               && after < !Define.conflict_burst_limit in
+       let newly_paused = before > 0.0 && after <= 0.0 in
        if !Define.conflict_rate_limit_is_aggregate then (
-               if before >= !Define.conflict_burst_limit
-               && after < !Define.conflict_burst_limit
+               if newly_penalised
                && after > 0.0
                then (
                        push dom doms.doms_with_conflict_penalty
-               ) else if before > 0.0 && after <= 0.0
+               ) else if newly_paused
                then (
                        let first_pause = Queue.is_empty doms.doms_conflict_paused in
                        push dom doms.doms_conflict_paused;
@@ -144,9 +166,12 @@ let decr_conflict_credit doms dom =
                ) else (
                        (* The queues are correct already: no further action needed. *)
                )
-       ) else if before > 0.0 && after <= 0.0 then (
-               doms.n_paused <- doms.n_paused + 1;
-               if doms.n_paused = 1 then doms.on_first_conflict_pause ()
+       ) else (
+               if newly_penalised then doms.n_penalised <- doms.n_penalised + 1;
+               if newly_paused then (
+                       doms.n_paused <- doms.n_paused + 1;
+                       if doms.n_paused = 1 then doms.on_first_conflict_pause ()
+               )
        )
 
 (* Give one point of credit to one domain, and update the queues appropriately. *)
@@ -175,9 +200,13 @@ let incr_conflict_credit doms =
                        let before = dom.Domain.conflict_credit in
                        let after = min (before +. 1.0) !Define.conflict_burst_limit in
                        dom.Domain.conflict_credit <- after;
+
                        if before <= 0.0 && after > 0.0
-                       then doms.n_paused <- doms.n_paused - 1
+                       then doms.n_paused <- doms.n_paused - 1;
+
+                       if before < !Define.conflict_burst_limit
+                       && after >= !Define.conflict_burst_limit
+                       then doms.n_penalised <- doms.n_penalised - 1
                in
-               (* Scope for optimisation (probably tiny): avoid iteration if all domains are at max credit *)
-               iter doms inc
+               if doms.n_penalised > 0 then iter doms inc
        )

diff --git a/tools/ocaml/xenstored/xenstored.ml b/tools/ocaml/xenstored/xenstored.ml
index d5c50fde52224a61ccff5133cedcb6aa156095ba..06387a80b11b8a0fc1968a27130747b31409d709 100644 (file)
--- a/tools/ocaml/xenstored/xenstored.ml
+++ b/tools/ocaml/xenstored/xenstored.ml
@@ -437,7 +437,10 @@ let _ =
                        peaceful_mw;
                let start_time = Unix.gettimeofday () in
                let timeout =
-                       let until_next_activity = min (max 0. (!next_frequent_ops -. start_time)) period_ops_interval in
+                       let until_next_activity =
+                               if Domains.all_at_max_credit domains
+                               then period_ops_interval
+                               else min (max 0. (!next_frequent_ops -. start_time)) period_ops_interval in
                        if peaceful_mw <> [] then 0. else until_next_activity
                in
                let inset, outset = Connections.select ~only_if:is_peaceful cons in