[PATCH] Minimize output fd lock time

Extract both trace and payload before acquiring the output descriptor lock.
The writes should take very shortly thus minimizing the hold time, the read
may take much longer.

diff --git a/blktrace.c b/blktrace.c
index b54bf7979fec6436a10d15d22c7d87735b6419c5..4f3c7b97697e80c1a3e760816e6b04c1950ff65e 100644 (file)
--- a/blktrace.c
+++ b/blktrace.c
@@ -182,7 +182,7 @@ static void stop_trace(void)
        }
 }
 
-static void extract_data(struct thread_information *tip, char *ofn, int nb)
+static void *extract_data(struct thread_information *tip, char *ofn, int nb)
 {
        int ret, bytes_left;
        unsigned char *buf, *p;
@@ -200,21 +200,14 @@ static void extract_data(struct thread_information *tip, char *ofn, int nb)
                                tip->cpu, tip->fn);
                        free(buf);
                        exit_trace(1);
+                       return NULL;
                } else {
                        p += ret;
                        bytes_left -= ret;
                }
        }
 
-       ret = write(tip->ofd, buf, nb);
-       if (ret != nb) {
-               perror(ofn);
-               fprintf(stderr,"Thread %d extract_data %s failed\n", tip->cpu, ofn);
-               free(buf);
-               exit_trace(1);
-       }
-
-       free(buf);
+       return buf;
 }
 
 static inline void tip_fd_unlock(struct thread_information *tip)
@@ -233,7 +226,7 @@ static void *extract(void *arg)
 {
        struct thread_information *tip = arg;
        int ret, pdu_len;
-       char dp[64];
+       char dp[64], *pdu_data;
        struct blk_io_trace t;
        pid_t pid = getpid();
        cpu_set_t cpu_mask;
@@ -256,6 +249,7 @@ static void *extract(void *arg)
                exit_trace(1);
        }
 
+       pdu_data = NULL;
        while (!is_done()) {
                ret = read(tip->fd, &t, sizeof(t));
                if (ret != sizeof(t)) {
@@ -281,6 +275,13 @@ static void *extract(void *arg)
 
                trace_to_be(&t);
 
+               if (pdu_len)
+                       pdu_data = extract_data(tip, dp, pdu_len);
+
+               /*
+                * now we have both trace and payload, get a lock on the
+                * output descriptor and send it off
+                */
                tip_fd_lock(tip);
 
                ret = write(tip->ofd, &t, sizeof(t));
@@ -290,11 +291,18 @@ static void *extract(void *arg)
                        exit_trace(1);
                }
 
-               if (pdu_len)
-                       extract_data(tip, dp, pdu_len);
+               if (pdu_data) {
+                       ret = write(tip->ofd, pdu_data, pdu_len);
+                       if (ret != pdu_len) {
+                               perror("write pdu data");
+                               exit_trace(1);
+                       }
 
-               tip_fd_unlock(tip);
+                       free(pdu_data);
+                       pdu_data = NULL;
+               }
 
+               tip_fd_unlock(tip);
                tip->events_processed++;
        }