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HDFS, mail # user - Re: HADOOP in Production


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Re: HADOOP in Production
Ted Dunning 2012-10-02, 23:13
On Tue, Oct 2, 2012 at 7:05 PM, Hank Cohen <[EMAIL PROTECTED]> wrote:

> There is an important difference between real time and real fast
>
> Real time means that system response must meet a fixed schedule.
> Real fast just means sooner is better.
>

Good thought, but real-time can also include a fixed schedule and a
specified list of exceptional conditions which would prevent meeting the
schedule.

It may also include a fixed schedule that must be met some fraction of the
time (usually very near 100% of the time).

Without providing exceptions, you basically force the designer to lie about
how reliable their system is.
> Real time systems always have hard schedules. The schedule could be in
> microseconds to control a laser for making masks for semiconductor
> manufacturing, milliseconds to control the ignition in your car or flight
> controls on an F-22 or  seconds for even slower moving processes.  In real
> time system missing the schedule can mean that very bad things happen:
> planes fall from the sky, your laser printer fries it's imaging drum,
> factories explode etc.
>

It can mean that.  But if you specify the exceptional situations you can
specifically mitigate for them.
> Most transaction processing is happy with real fast
> The folks doing high velocity trading are pretty close to real time but
> they probably will be happy with real fast.
> If real fast systems miss a schedule then someone loses money.
>

Yeah.  And if you talk to these guys, they know the difference and ask for
real-time.

> The reason that RTOS type operating systems are popular for real time
> applications is that they don't allow operations to spend indeterminate
> amounts of time in uninterruptable states.  Java will never qualify as a
> real time system because it has garbage collection and garbage collection
> can lock up a system for an indefinite amount of time while it goes through
> marking and counting.
>

You are behind the times on a few counts.

- Java's collectors don't "count".

- Java can be real-time:

http://rtjava.blogspot.co.uk/2009/07/real-time-java-vms.html

- Garbage collection can be deterministic and real-time:

http://www.ibm.com/developerworks/java/library/j-rtj4/index.html
http://docs.oracle.com/cd/E13221_01/wlrt/docs30/intro_wlrt/tuning.html
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Gauthier, Alexander 2012-10-03, 00:15