[ovs-dev] [PATCH v2 2/2] Minimize the number of time calls in time_poll()

[anton.ivanov at cambridgegreys.com]

From: Anton Ivanov <anton.ivanov at cambridgegreys.com>

time_poll() makes an excessive number of time_msec() calls
which incur a performance penalty.

1. Avoid time_msec() call for timeout calculation when time_poll()
is asked to skip poll()

2. Reuse the time_msec() result from deadline calculation for
last_wakeup and timeout calculation.

Signed-off-by: Anton Ivanov <anton.ivanov at cambridgegreys.com>
---
 lib/timeval.c | 36 +++++++++++++++++++++---------------
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/lib/timeval.c b/lib/timeval.c
index c6ac87376..64ab22e05 100644
--- a/lib/timeval.c
+++ b/lib/timeval.c
@@ -287,7 +287,7 @@ time_poll(struct pollfd *pollfds, int n_pollfds, HANDLE *handles OVS_UNUSED,
           long long int timeout_when, int *elapsed)
 {
     long long int *last_wakeup = last_wakeup_get();
-    long long int start;
+    long long int start, now;
     bool quiescent;
     int retval = 0;
 
@@ -297,28 +297,31 @@ time_poll(struct pollfd *pollfds, int n_pollfds, HANDLE *handles OVS_UNUSED,
     if (*last_wakeup && !thread_is_pmd()) {
         log_poll_interval(*last_wakeup);
     }
-    start = time_msec();
+    now = start = time_msec();
 
     timeout_when = MIN(timeout_when, deadline);
     quiescent = ovsrcu_is_quiescent();
 
     for (;;) {
-        long long int now = time_msec();
         int time_left;
 
-        if (now >= timeout_when) {
+        if (n_pollfds == 0) {
             time_left = 0;
-        } else if ((unsigned long long int) timeout_when - now > INT_MAX) {
-            time_left = INT_MAX;
         } else {
-            time_left = timeout_when - now;
-        }
-
-        if (!quiescent) {
-            if (!time_left) {
-                ovsrcu_quiesce();
+            if (now >= timeout_when) {
+                time_left = 0;
+            } else if ((unsigned long long int) timeout_when - now > INT_MAX) {
+                time_left = INT_MAX;
             } else {
-                ovsrcu_quiesce_start();
+                time_left = timeout_when - now;
+            }
+
+            if (!quiescent) {
+                if (!time_left) {
+                    ovsrcu_quiesce();
+                } else {
+                    ovsrcu_quiesce_start();
+                }
             }
         }
 
@@ -329,6 +332,8 @@ time_poll(struct pollfd *pollfds, int n_pollfds, HANDLE *handles OVS_UNUSED,
          */
         if (n_pollfds != 0) {
             retval = poll(pollfds, n_pollfds, time_left);
+        } else {
+            retval = 0;
         }
         if (retval < 0) {
             retval = -errno;
@@ -355,7 +360,8 @@ time_poll(struct pollfd *pollfds, int n_pollfds, HANDLE *handles OVS_UNUSED,
             ovsrcu_quiesce_end();
         }
 
-        if (deadline <= time_msec()) {
+        now = time_msec();
+        if (deadline <= now) {
 #ifndef _WIN32
             fatal_signal_handler(SIGALRM);
 #else
@@ -372,7 +378,7 @@ time_poll(struct pollfd *pollfds, int n_pollfds, HANDLE *handles OVS_UNUSED,
             break;
         }
     }
-    *last_wakeup = time_msec();
+    *last_wakeup = now;
     refresh_rusage();
     *elapsed = *last_wakeup - start;
     return retval;
-- 
2.20.1