Re: [ovs-dev] [PATCH v3 2/2] Minimize the number of time calls in time_poll()

[Ben Pfaff]

On Fri, Jul 16, 2021 at 02:42:33PM +0100, anton.iva...@cambridgegreys.com wrote:
> From: Anton Ivanov 
> 
> 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 

I'll take another look at this in v4.
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[ovs-dev] [PATCH v3 2/2] Minimize the number of time calls in time_poll()

[anton . ivanov]

From: Anton Ivanov 

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 
---
 lib/timeval.c | 36 +---
 1 file changed, 21 insertions(+), 15 deletions(-)

diff --git a/lib/timeval.c b/lib/timeval.c
index 42d8c258c..e10a6667a 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

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