INFO now contains the time (in usecs) needed to fork() in order to persist. The info is available in the stats section of INFO.

src/aof.c

@@ -589,13 +589,15 @@ werr:
  */
 int rewriteAppendOnlyFileBackground(void) {
     pid_t childpid;
+    long long start;
 
     if (server.bgrewritechildpid != -1) return REDIS_ERR;
     if (server.vm_enabled) waitEmptyIOJobsQueue();
+    start = ustime();
     if ((childpid = fork()) == 0) {
-        /* Child */
         char tmpfile[256];
 
+        /* Child */
         if (server.vm_enabled) vmReopenSwapFile();
         if (server.ipfd > 0) close(server.ipfd);
         if (server.sofd > 0) close(server.sofd);
@@ -607,6 +609,7 @@ int rewriteAppendOnlyFileBackground(void) {
         }
     } else {
         /* Parent */
+        server.stat_fork_time = ustime()-start;
         if (childpid == -1) {
             redisLog(REDIS_WARNING,
                 "Can't rewrite append only file in background: fork: %s",

src/rdb.c

@@ -498,10 +498,12 @@ werr:
 
 int rdbSaveBackground(char *filename) {
     pid_t childpid;
+    long long start;
 
     if (server.bgsavechildpid != -1) return REDIS_ERR;
     if (server.vm_enabled) waitEmptyIOJobsQueue();
     server.dirty_before_bgsave = server.dirty;
+    start = ustime();
     if ((childpid = fork()) == 0) {
         /* Child */
         if (server.vm_enabled) vmReopenSwapFile();
@@ -514,6 +516,7 @@ int rdbSaveBackground(char *filename) {
         }
     } else {
         /* Parent */
+        server.stat_fork_time = ustime()-start;
         if (childpid == -1) {
             redisLog(REDIS_WARNING,"Can't save in background: fork: %s",
                 strerror(errno));

src/redis.c

@@ -909,6 +909,7 @@ void initServer() {
     server.stat_keyspace_misses = 0;
     server.stat_keyspace_hits = 0;
     server.stat_peak_memory = 0;
+    server.stat_fork_time = 0;
     server.unixtime = time(NULL);
     aeCreateTimeEvent(server.el, 1, serverCron, NULL, NULL);
     if (server.ipfd > 0 && aeCreateFileEvent(server.el,server.ipfd,AE_READABLE,
@@ -1207,6 +1208,7 @@ sds genRedisInfoString(void) {
         "hash_max_zipmap_value:%zu\r\n"
         "pubsub_channels:%ld\r\n"
         "pubsub_patterns:%u\r\n"
+        "latest_fork_usec:%lld\r\n"
         "vm_enabled:%d\r\n"
         "role:%s\r\n"
         ,REDIS_VERSION,
@@ -1249,6 +1251,7 @@ sds genRedisInfoString(void) {
         server.hash_max_zipmap_value,
         dictSize(server.pubsub_channels),
         listLength(server.pubsub_patterns),
+        server.stat_fork_time,
         server.vm_enabled != 0,
         server.masterhost == NULL ? "master" : "slave"
     );

src/redis.h

@@ -401,6 +401,7 @@ struct redisServer {
     long long stat_keyspace_hits;   /* number of successful lookups of keys */
     long long stat_keyspace_misses; /* number of failed lookups of keys */
     size_t stat_peak_memory;        /* max used memory record */
+    long long stat_fork_time;       /* time needed to perform latets fork() */
     /* Configuration */
     int verbosity;
     int maxidletime;

src/util.c

@@ -5,6 +5,7 @@
 #include <ctype.h>
 #include <limits.h>
 #include <math.h>
+#include <sys/time.h>
 
 #include "util.h"