From: Scott Cheloha <scottcheloha@gmail.com>
Subject: Re: systat(1): iostat: compute rates with elapsed time
To: tech@openbsd.org
Date: Mon, 8 Apr 2024 22:44:50 -0500

On Thu, Apr 04, 2024 at 02:18:54PM -0500, Scott Cheloha wrote:
> In TIME mode, the iostat view sets etime to naptime before computing
> rates.  This is wrong.  The elapsed time is never exactly naptime.
> 
> Instead, measure the real elapsed time between each print_io() call.
> We need to read the clock during every call, regardless of mode,
> because the mode can change between updates.
> 
> This patch also changes the iostat view's behavior during the first
> update.  During that first update, iostat has no prior data to use
> when computing rates.  Currently, the resulting output is "totals
> since boot divided by naptime".  Here's a concrete example:
> 
> $ systat -d1 io 1
> DEVICE          READ    WRITE     RTPS    WTPS     SEC
> sd0         4544872K 7316640K   365102  687727   101.9
> sd1                0        0        0       0     0.0
> Totals      4544872K 7316640K   365102  687727   101.9
> $ systat -d1 io 2
> DEVICE          READ    WRITE     RTPS    WTPS     SEC
> sd0         2272436K 3658335K   182551  343866    50.9
> sd1                0        0        0       0     0.0
> Totals      2272436K 3658335K   182551  343866    50.9
> $ systat -d io 0.5
> DEVICE          READ    WRITE     RTPS    WTPS     SEC
> sd0         9089744K   14295M   730204 1376152   204.7
> sd1                0        0        0       0     0.0
> Totals      9089744K   14295M   730204 1376152   204.7
> 
> The second output is half the first (1 / 2).  The third output is
> twice the first (1 / 0.5).  This is difficult to explain, let alone
> document.  The numbers are also meaningless.
> 
> IMHO, if we're going to print something during that first update, it
> makes more sense to unconditionally set etime to 1.0 and print the
> totals.
> 
> Thoughts?  ok?

Ping.

Index: iostat.c
===================================================================
RCS file: /cvs/src/usr.bin/systat/iostat.c,v
diff -u -p -r1.50 iostat.c
--- iostat.c	14 Aug 2021 14:22:26 -0000	1.50
+++ iostat.c	4 Apr 2024 19:15:29 -0000
@@ -166,13 +166,23 @@ read_io(void)
 void
 print_io(void)
 {
+	static struct timespec prev;
+	struct timespec elapsed, now;
+	static int first = 1;
 	int n, count = 0;
 	int curr;
 
-	if (state == BOOT)
+	clock_gettime(CLOCK_MONOTONIC, &now);
+	if (first) {
+		first = 0;
 		etime = 1.0;
-	else
-		etime = naptime;
+	} else if (state == BOOT) {
+		etime = 1.0;
+	} else {
+		timespecsub(&now, &prev, &elapsed);
+		etime = elapsed.tv_sec + elapsed.tv_nsec / 1000000000.0;
+	}
+	prev = now;
 
 	/* XXX engine internals: save and restore curr_line for bcache */
 	curr = curr_line;