Benchmark.t: test for inconsistent clock

test 15 has been failing intermittently in smokes for ages now.
This does countit(3, ...) and countit(1, ...) and checks that the first
takes approx three times longer than the second.

This commit adds in near the beginning a crude timing loop that directly
uses times() rather than anything in the Benchmark module, and that does
approx 1s and 3s of loops, and if the results aren't consistent, sets a
global flag, $INCONSISTENT_CLOCK, that causes timing-sensitive tests to be
skipped.

For now it only skips test 15. If this is successful, I'll look to
expand it to other failing tests like 128/129.

1 files changed, 63 insertions, 2 deletions

diff --git a/lib/Benchmark.t b/lib/Benchmark.t
index 004092e1e9..571b9350b2 100644
--- a/lib/Benchmark.t
+++ b/lib/Benchmark.t
@@ -47,9 +47,65 @@ timeit( 1, sub { $foo = @_ });
 is ($foo, 0, "benchmarked code called without arguments");
 
 
-print "# Burning CPU to benchmark things will take time...\n";
+print "# Burning CPU to see if clock is consistent...\n";
 
+# Run some code for approx 3 secs, then for 1 sec. If the first doesn't
+# take appoex 3 times longer than the second, then skip any tests which
+# require a consistent clock
 
+my $INCONSISTENT_CLOCK = 0;
+
+{
+    my ($t0, $t1, $tdelta);
+
+    $t0 = times; 1 while times == $t0; # wait for OS clock to tick
+
+    # guess approx n for code to run for 1 sec
+    my $n = 1;
+    while ($n < 1_000_000_000) { # eventually stop in worst case
+	$t0 = times;
+	fib($ballast) for 1..$n;
+	$t1 = times;
+	$tdelta = ($t1 - $t0);
+	last if ($tdelta) >= 1.0;
+	$n *= 2;
+    }
+    print "# did $n iterations in $tdelta sec\n";
+
+    # adjust n for exactly one second
+    $n /= $tdelta;
+
+    # now run enough loops for approx 3 secs
+
+    $t0 = times; 1 while times == $t0; # wait for OS clock to tick
+    $t0 = times;
+    fib($ballast) for 1..($n*3);
+    $t1 = times;
+    my $td3 = ($t1 - $t0);
+    print "# approx 3 sec delta is $td3 secs\n";
+
+    # now run enough loops for approx 1 sec
+
+    $t0 = times; 1 while times == $t0; # wait for OS clock to tick
+    $t0 = times;
+    fib($ballast) for 1..$n;
+    $t1 = times;
+    my $td1 = ($t1 - $t0);
+    print "# approx 1 sec delta is $td1 secs\n";
+
+    # we use 0.7 of $delta so that we err on the side of assuming
+    # a bad clock and skip tests; otherwise we might be just within the
+    # delta here, and just outside the delta on tests, and so get random
+    # failures
+    if ( abs(($td3 - 3*$td1) / $td3) > 0.7*$delta) {
+	print "# INCONSISTENT CLOCK! - will skip timing-related tests\n";
+	$INCONSISTENT_CLOCK = 1;
+    }
+
+}
+
+
+print "# Burning CPU to benchmark things; will take time...\n";
 
 # We need to do something fairly slow in the coderef.
 # Same coderef. Same place in memory.
@@ -89,7 +145,11 @@ print "# in_onesec_adj=$in_onesec_adj adjusted iterations\n";
 {
   my $difference = $in_onesec_adj - $estimate;
   my $actual = abs ($difference / $in_onesec_adj);
-  cmp_ok($actual, '<=', $delta, "is $in_onesec_adj within $delta of estimate ($estimate)")
+  SKIP: {
+    skip("INCONSISTENT CLOCK") if $INCONSISTENT_CLOCK;
+
+    cmp_ok($actual, '<=', $delta,
+		"is $in_onesec_adj within $delta of estimate ($estimate)")
     or do {
 	diag("  in_threesecs     = $in_threesecs");
 	diag("  in_threesecs_adj = $in_threesecs_adj");
@@ -101,6 +161,7 @@ print "# in_onesec_adj=$in_onesec_adj adjusted iterations\n";
 	diag("  cpu1             = $cpu1");
 	diag("  sys1             = $sys1");
     };
+  }
 }
 
 # I found that the eval'ed version was 3 times faster than the coderef.