Looks good to me. I built PG with this change, no kernel warnings after
~10 minutes of running. I'll continue to monitor but I think this fixes
the syndrome. Thanks Tom.
-Greg
On Fri, 18 Nov 2011, Tom Lane wrote:
Claudio Freire <klaussfreire@xxxxxxxxx> writes:
On Thu, Nov 17, 2011 at 10:07 PM, Greg Matthews
<gregory.a.matthews@xxxxxxxx> wrote:
if (smoothed_alloc <= (float) recent_alloc)
smoothed_alloc = recent_alloc;
else if (smoothed_alloc >= 0.00001)
smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
smoothing_samples;
I don't think that logic is sound.
Rather,
if (smoothed_alloc <= (float) recent_alloc) {
smoothed_alloc = recent_alloc;
} else {
if (smoothed_alloc < 0.000001)
smoothed_alloc = 0;
smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
smoothing_samples;
}
The real problem with either of these is the cutoff number is totally
arbitrary. I'm thinking of something like this:
/*
* Track a moving average of recent buffer allocations. Here, rather than
* a true average we want a fast-attack, slow-decline behavior: we
* immediately follow any increase.
*/
if (smoothed_alloc <= (float) recent_alloc)
smoothed_alloc = recent_alloc;
else
smoothed_alloc += ((float) recent_alloc - smoothed_alloc) /
smoothing_samples;
/* Scale the estimate by a GUC to allow more aggressive tuning. */
upcoming_alloc_est = smoothed_alloc * bgwriter_lru_multiplier;
+ /*
+ * If recent_alloc remains at zero for many cycles,
+ * smoothed_alloc will eventually underflow to zero, and the
+ * underflows produce annoying kernel warnings on some platforms.
+ * Once upcoming_alloc_est has gone to zero, there's no point in
+ * tracking smaller and smaller values of smoothed_alloc, so just
+ * reset it to exactly zero to avoid this syndrome.
+ */
+ if (upcoming_alloc_est == 0)
+ smoothed_alloc = 0;
/*
* Even in cases where there's been little or no buffer allocation
* activity, we want to make a small amount of progress through the buffer
regards, tom lane
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