oxenstored: reduce syslog call overhead

We noticed that, if configured to use syslog as the logging backend, every
single line of access logging (via the syslog C binding) will call stat on
/etc/localtime for 3 times. The rational behind this is probably to detect any
timezone changes over time.

This is a considerable cost we'd like to avoid, given the intensiveness of our
access logging --- we log almost every xenstore status change (for good
reason). Also a running Xen host is rarely a mobile environment, so the little
benefit can hardly justify the cost.

Setting up the TZ environment varialbe can avoid stat calls.

Signed-off-by: Zheng Li <dev@zheng.li>
Reviewed-by: David Scott <dave.scott@citrix.com>

diff --git a/tools/ocaml/xenstored/logging.ml b/tools/ocaml/xenstored/logging.ml
index 3d4a3292fd1ced09d14bd6f0f8910a7b729b2a56..665b922419655fa80c2e84dc75273e5e463ee1bc 100644 (file)
--- a/tools/ocaml/xenstored/logging.ml
+++ b/tools/ocaml/xenstored/logging.ml
@@ -130,8 +130,17 @@ let string_of_date () =
                tm.Unix.tm_hour tm.Unix.tm_min tm.Unix.tm_sec
                (int_of_float (1000.0 *. msec))
 
+(* We can defer to syslog for log management *)
 let make_syslog_logger facility =
-       (* We defer to syslog for log management *)
+       (* When TZ is unset in the environment, each syslog call will stat the
+          /etc/localtime file at least three times during the process. We'd like to
+          avoid this cost given that we are not a mobile environment and we log
+          almost every xenstore entry update/watch. *)
+       let () =
+               let tz_is_set =
+                       try String.length (Unix.getenv "TZ") > 0
+                       with Not_found -> false in
+               if not tz_is_set then Unix.putenv "TZ" "/etc/localtime" in
        let nothing () = () in
        let write ?level s =
                let level = match level with