diff -u ureadahead-0.100.0/src/trace.c ureadahead-0.100.0/src/trace.c
--- ureadahead-0.100.0/src/trace.c
+++ ureadahead-0.100.0/src/trace.c
@@ -117,6 +117,7 @@
        int timeout)
 {
 	int                 dfd;
+	FILE                *fp;
 	int                 unmount = FALSE;
 	int                 old_sys_open_enabled = 0;
 	int                 old_open_exec_enabled = 0;
@@ -129,6 +130,7 @@
 	struct timeval      tv;
 	nih_local PackFile *files = NULL;
 	size_t              num_files = 0;
+	size_t              num_cpus = 0;
 
 	/* Mount debugfs if not already mounted */
 	dfd = open (PATH_DEBUGFS "/tracing", O_RDONLY | O_NOATIME);
@@ -149,7 +151,29 @@
 		unmount = TRUE;
 	}
 
-	/* Enable tracing of open() syscalls */
+	/*
+	 * Count the number of CPUs, default to 1 on error. 
+	 */
+	fp = fopen("/proc/cpuinfo", "r");
+	if (fp) {
+		int line_size=1024;
+		char *processor="processor";
+		char *line = malloc(line_size);
+		if (line) {
+			num_cpus = 0;
+			while (fgets(line,line_size,fp) != NULL) {
+				if (!strncmp(line,processor,strlen(processor)))
+					num_cpus++;
+			}
+			free(line);
+			nih_message("Counted %d CPUs\n",num_cpus);
+		}
+		fclose(fp);
+	}
+	if (!num_cpus)
+		num_cpus = 1;
+
+    /* Enable tracing of open() syscalls */
 	if (set_value (dfd, "events/fs/do_sys_open/enable",
 		       TRUE, &old_sys_open_enabled) < 0)
 		goto error;
@@ -166,7 +190,7 @@
 
 		old_uselib_enabled = -1;
 	}
-	if (set_value (dfd, "buffer_size_kb", 128000, &old_buffer_size_kb) < 0)
+	if (set_value (dfd, "buffer_size_kb", 128000/num_cpus, &old_buffer_size_kb) < 0)
 		goto error;
 	if (set_value (dfd, "tracing_enabled",
 		       TRUE, &old_tracing_enabled) < 0)
diff -u ureadahead-0.100.0/debian/changelog ureadahead-0.100.0/debian/changelog
--- ureadahead-0.100.0/debian/changelog
+++ ureadahead-0.100.0/debian/changelog
@@ -1,3 +1,15 @@
+ureadahead (0.100.0-4.1.3) lucid-proposed; urgency=low
+
+  * Count the number of CPUs and divide buffer_size_kb by the number of CPUs.
+    Users should watch for instances of "mmiotrace has lost events" in dmesg to make
+    sure the trace buffers are not too small. The original value for
+    buffer_size_kb was chosen somewhat arbitrarily. Empirical testing
+    has shown that its large enough, so we don't actually know where the lower
+    boundary lies.
+    -LP: #491943
+
+ -- Tim Gardner <tim.gardner@canonical.com>  Fri, 13 Aug 2010 12:44:12 -0700
+
 ureadahead (0.100.0-4.1.2) lucid-proposed; urgency=low
 
   * Restore buffer_size_kb upon exit, but do it _after_