CA-409482: Using computed delay for RRD loop

RRD loop is executed each 5 seconds. It delays fixed 5 seconds between each
loop. But the loop self also consumes time (The time consuming depends on CPU's
count. If there are many CPUs, the time consuming may be hundreds milliseconds).
This implementation leads RRD will take an offset after several loops. Then one
of RRD data lose and a gap can be observed on XenCenter performance graph.

The solution is to use computed delay (timeslice - loop time consuming) instead
of fixed delay.

Signed-off-by: Bengang Yuan <[email protected]>

Author: BengangY

ocaml/xcp-rrdd/bin/rrdd/xcp_rrdd.ml

@@ -537,17 +537,30 @@ let monitor_write_loop writers =
       Xenctrl.with_intf (fun xc ->
           while true do
             try
+              let last_loop_start_time = Unix.gettimeofday () in
               do_monitor_write xc writers ;
               with_lock Rrdd_shared.last_loop_end_time_m (fun _ ->
                   Rrdd_shared.last_loop_end_time := Unix.gettimeofday ()
               ) ;
-              Thread.delay !Rrdd_shared.timeslice
+              (* Using computed delay (timeslice - loop time consuming) instead
+                 of fixed delay*)
+              let delay =
+                !Rrdd_shared.timeslice
+                -. (!Rrdd_shared.last_loop_end_time -. last_loop_start_time)
+              in
+              if delay > 0.0 then
+                Thread.delay delay
+              else
+                warn
+                  "%s: Monitor write loop took so long time that the delay \
+                   (%f) is less than 0, so skip the delay"
+                  __FUNCTION__ delay
             with e ->
               Backtrace.is_important e ;
               warn
-                "Monitor/write thread caught an exception. Pausing for 10s, \
-                 then restarting: %s"
-                (Printexc.to_string e) ;
+                "%s: Monitor/write thread caught an exception. Pausing for \
+                 10s, then restarting: %s"
+                __FUNCTION__ (Printexc.to_string e) ;
               log_backtrace e ;
               Thread.delay 10.
           done