udevd: implement a more efficient queue file format

Directory lookups show up in profiling. The queue files are responsible
for a large proportion of file-related system calls in udev coldplug.

Instead of creating a file for each event, append their details to a
log file.  The file is periodically rebuilt (garbage-collected) to
prevent it from growing indefinitely.

This single queue file replaces both the queue directory and the
uevent_seqnum file. On desktop systems the file tends not to grow
beyond one page. So it should also save a small amount of memory in
tmpfs.

Tests on a running EeePC indicate average savings of 5% *udevd* cpu time
as measured by oprofile. __link_path_walk is reduced from 1.5% to
1.3%. It is not completely clear where the rest of the gains come from.

In tests running ~400 events, the queue file is rebuilt about 5 times.

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>

1 files changed, 7 insertions, 15 deletions

diff --git a/udev/udevadm-settle.c b/udev/udevadm-settle.c
index 52d9c0b244..f1052aa148 100644
--- a/udev/udevadm-settle.c
+++ b/udev/udevadm-settle.c
@@ -173,24 +173,16 @@ int udevadm_settle(struct udev *udev, int argc, char *argv[])
 	}
 
 	while (!is_timeout) {
-		/* exit if queue is empty */
-		if (udev_queue_get_queue_is_empty(udev_queue))
-			break;
-
-		/* if asked for, wait for a specific sequence of events */
 		if (start > 0) {
-			unsigned long long seq;
-			int finished;
-
-			finished = 0;
-			for (seq = start; seq <= end; seq++) {
-				finished  = udev_queue_get_seqnum_is_finished(udev_queue, seq);
-				if (!finished)
-					break;
-			}
-			if (finished)
+			/* if asked for, wait for a specific sequence of events */
+			if (udev_queue_get_seqnum_sequence_is_finished(udev_queue, start, end) == 1)
+				break;
+		} else {
+			/* exit if queue is empty */
+			if (udev_queue_get_queue_is_empty(udev_queue))
 				break;
 		}
+
 		usleep(1000 * 1000 / LOOP_PER_SECOND);
 	}