entropy big drops

[trk7]

Hi,

I am using it on a esxi host running debian 7.0. udev rules are as per the documentation.
rngd is running this way:
rngd -r /dev/TrueRNG –fill-watermark=100% –feed-interval=1 –rng-quality=low –random-step=1024
(been fiddling with the configs)
I am seeing big drops in the available entropy (soon filled though), data is below . Is that normal? I was expecting to see some smooth graphs as in your product page. I will try on a Ubuntu machine that is not virtualized to rule out esxi causing any issue.

time available entropy
16:50:41 3840
16:50:42 3840
16:50:43 3840
16:50:44 3840
16:50:45 3841
16:50:46 3840
16:50:47 3840
16:50:48 3840
16:50:49 3840
16:50:50 3840
16:50:51 3840
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16:51:00 3840
16:51:01 3584
16:51:02 3585
16:51:03 3840
16:51:04 3584
16:51:05 3840
16:51:06 3840
16:51:07 3840
16:51:08 3840
16:51:09 3840
16:51:10 3840
16:51:11 3584
16:51:12 3840
16:51:13 3840
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16:51:15 3840
16:51:16 3840
16:51:17 3840
16:51:18 3840
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16:51:20 3840
16:51:21 3840
16:51:22 1025
16:51:23 2817
16:51:25 3840
16:51:26 3840
16:51:27 3840
16:51:28 2561
16:51:29 2561
16:51:30 3840
16:51:31 3840
16:51:32 3584
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16:52:46 3584
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16:52:49 3840
16:52:50 130
16:52:51 2305
16:52:52 3840
16:52:53 513
16:52:54 1793
16:52:55 1025
16:52:56 1026
16:52:57 130
16:52:58 2304
16:52:59 2561
16:53:00 3840
16:53:01 2561
16:53:02 130
16:53:03 2305
16:53:04 3840
16:53:05 2304
16:53:06 3840
16:53:07 3584
16:53:08 2049
16:53:09 3840
16:53:10 769
16:53:11 3840
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16:53:14 2304
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16:53:17 1025
16:53:18 2304
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16:53:24 1793
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16:53:57 3330
16:53:58 1035
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16:54:01 2560
16:54:02 1793
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16:54:04 2058
16:54:05 2049
16:54:06 513
16:54:07 2050
16:54:08 2314
16:54:09 3840
16:54:10 3840
16:54:11 2304
16:54:12 1025
16:54:13 3840
16:54:14 2314
16:54:15 3840
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16:54:17 2304
16:54:18 3840
16:54:19 2049
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16:54:22 2059
16:54:23 3840
16:54:24 3840
16:54:25 130
16:54:26 2561
16:54:27 3840
16:54:28 3840
16:54:29 2049
16:54:30 2304
16:54:31 2573
16:54:32 2305
16:54:33 2313
16:54:34 3840
16:54:35 2060
16:54:36 769
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16:54:38 2313
16:54:39 3840
16:54:40 2305
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16:54:43 1290
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16:54:46 3776
16:54:47 2570
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16:54:49 2305
16:54:50 130
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16:54:54 769
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16:54:56 2305
16:54:57 3840
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16:55:00 2049
16:55:01 2433
16:55:02 1664
16:55:03 1872
16:55:04 141
16:55:05 138
16:55:06 3334
16:55:07 3584
16:55:08 3840
16:55:09 257
16:55:10 3840
16:55:11 3840
16:55:12 3840
16:55:13 2305
16:55:14 3840
16:55:15 3840
16:55:16 2304
16:55:17 2305
16:55:18 130
16:55:19 3840
16:55:20 3328
16:55:21 3840
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16:55:37 3584
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16:55:40 780
16:55:41 2313
16:55:42 514
16:55:43 3840
16:55:44 2305
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16:55:46 3840
16:55:47 513
16:55:48 3840
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16:56:00 2049
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16:56:04 3840
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16:56:06 2569
16:56:07 3584
16:56:08 3840
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16:56:13 780
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16:56:18 769
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16:56:20 2048
16:56:21 2561
16:56:22 3840
16:56:23 130
16:56:24 3840
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16:56:26 769
16:56:27 1025
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16:56:29 2305
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16:56:33 257
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16:56:36 2314
16:56:37 3840
16:56:38 769
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16:56:40 2049
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16:56:43 2305
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16:56:47 4096
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[Ubld.it Staff]

This looks pretty normal to me. From the TrueRNG side (hardware), it is generating plenty of entropy. However, there are other factors at play. First off according to your settings, rngd is only filling once per second. So thats why on most of the dips, entropy goes down, then back up. If rngd was more responsive it would theoretically attempt to fill faster. I think rngd’s lowest interval is 1 second, unless you omit the option and let it force feed the pool as fast as it can.

Here is a snippet from the ubuntu man page of rngd.

-t n, –feed-interval=n (default: 60)
If feed-interval is not zero, rngd will force-feed entropy to
the random device even when the entropy pool is full every feed-
interval seconds. Setting this too low may cause rngd to
dominate the contents of the entropy pool.
.

Second is the kernel it’s self. Just because rngd tries to give it entropy, doesn’t mean the kernel has to take it. This algorithm of accepting outside entropy (to mix with naturally occuring hardware event entropy) varies from kernel version to kernel version and distribution.

For the most part I can say what I’m seeing if I plot these numbers on a spreadsheet is normal. For reference, I believe our graph was taken on a Ubuntu machine without using the feed interval argument.

[Author]

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