linux-kernel - Re: random: Benchamrking fast

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Date:	14 Jun 2014 04:03:29 -0400
From:	"George Spelvin" <linux@...izon.com>
To:	linux@...izon.com, tytso@....edu
Cc:	hpa@...ux.intel.com, linux-kernel@...r.kernel.org,
	mingo@...nel.org, price@....edu
Subject: Re: random: Benchamrking fast_mix2

> Want to give this patch a try on your collection of machines?

Not until I fix it to 

+#if ADD_INTERRUPT_BENCH
+static unsigned long avg_cycles;
+
+#define AVG_SHIFT 8	/* Exponential average factor k=1/256 */
+#define FIXED_1_2 (1 << (AVG_SHIFT-1))
+
+static void add_interrupt_bench(cycles_t start)
+{
+	cycles_t duration = random_get_entropy() - start;
+
+	/* Use a weighted moving average */
+	avg_cycles += ((avg_cycles + FIXED_1_2) >> AVG_SHIFT) - duration;
+}
+#else
+#define add_interrupt_bench(x)
+#endif
+

... because the way you did it was just silly.

See net/ipv4/tcp_input.c:tcp_rtt_estimator().

That also shows you how to add a mean deviation estimator, too.
I'll go do that now...
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