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Mark,<br>
<br>
This issue of how random number generators are initialized deserves
a more general discussion, leading to an agreed policy, I think. I
also ran into this recently when I submitted a couple thousand
parallel simulation jobs on the OSG, and about 30% of them ended up
duplicating a seed used by another job. Again, I traced it down to
the outdated practice of using unix time() to seed the random number
generator. I say "outdated", although I think there never really
was a time when it made sense to do that, except for
run-once-and-throw-away software projects.<br>
<br>
All of our applications that use random numbers should have a
command-line option for passing in the seed, but that only begs the
question. What is a good source of seeds for Gluex software in need
of pseudo-random sequences? I can think of several good options we
might consider:<br>
<ol>
<li>/dev/random - this is a pretty good attempt by the linux
kernel folks to provide us with true random numbers, for things
like secure key generation where it really matters. It collects
entropy from the OS environment and keeps track of how much
entropy it spends each time it makes a random number for you.
The down side is that you can easily drain the entropy pool,
after which your reads might wait for quite a while for the
entropy to build up again, depending on how busy the system is.
You open and read from /dev/random just like any file, but don't
read more bytes than you are going to use, because it depletes
the pool.<br>
</li>
<li>/dev/urandom - this behaves like /dev/random, except that it
never blocks. If it runs low in entropy then it just continues
to supply pseudo-random numbers, in the spirit of the
pseudo-random number generators used internally by our software.</li>
<li>a web service, similar to the one provided by
<a href="http://www.random.org">http://www.random.org</a>. Of course we do not require all of the
statistical properties guaranteed by random.org, and we don't
want to pay their fees. But we could make our own web service
hosted on a jlab server and replicated at other sites to ensure
high availability, and make a command-line tool that grabs a
given number of bytes and spit them out on standard out. Using
this service might look something like "$ genr8 -s
`get_gluex_randoms -n 2 -f '%d' ` ..." <br>
</li>
</ol>
If one wanted to be able to run "off the grid", the command
"get_gluex_randoms" could fail over to /dev/random or /dev/urandom
if the internet could not be reached.<br>
<br>
Comments?<br>
<br>
-Richard J.<br>
<br>
<br>
<br>
<br>
<br>
<br>
On 12/3/2010 8:25 AM, David Lawrence wrote:
<blockquote type="cite" cite="mid:4CF8EFBE.3090508@jlab.org">
<pre wrap="">
Hi Kei,
I'll modify the program to allow the seed to be set explicitly via
command line argument. I'll let you know when the modification is available.
Regards,
-David
On 12/2/10 4:45 PM, Kei Moriya wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Dear Offliners,
I have been working on the Hall D simulation packages,
and have encountered an issue with genr8, and was wondering
if anybody could help me.
The issue I encountered is that the seed used for the
random generator in genr8 is the current time,
so if the program is run consecutively within the same
second, the generated result is exactly the same, and I end
up with several copies of the same files.
I think that the use of the current time should be avoided,
not just because of the behavior above, but also because
it becomes impossible to reproduce the same file at a later
time if necessary.
I think passing the seed in as another runtime parameter
to genr8 would require only a few lines of extra code.
Could somebody do this for me, or point me to how to update
the program myself?
Thanks in advance.
Kei Moriya
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