Re: USN-2124-1 (OpenJDK-6) has regressed..?
Gene Heskett <gheskett <at> wdtv.com>
2014-03-04 19:51:01 GMT
On Tuesday 04 March 2014 14:08:51 William Scott Lockwood III did opine:
> On Tue, Mar 4, 2014 at 10:21 AM, Gene Heskett <gheskett <at> wdtv.com> wrote:
> > Its the distribution that linuxcnc is built to run on. We will have
> > that fixed before 14.06 I'm sure but its not at all easy to make
> > linux into a deterministic real time system. We need a heartbeat
> > with a timing jitter under 5 u-secs. Current kernels are 3
> > magnitudes worse.
> Have you tried the real time kernel?
I have not personally. Several of the developers have but its pointed at
desktop stuff and is not a lot better than stock for background stuff, and
the heart of linuxcnc is the background process that handles the I/O. The
most promising patch kit other than RTAI, is the xenomai patch kit, that I
have built, on an earlier kernel whose base version I do not recall ATM,
and if it was the best there was available, we would probably use it, at
the cost of the top half of the machines speed capability. Or accuracy as
its a trade off between the two.
Amazingly, this whole low end of linuxcnc, running stepper motors to move
the machines as oppsed to the 2 to 4x more costly servo motors, appears to
be able to run rather nicely on the Beaglebone Black with its huge bank of
PRU's that run asynch with the kernel, each on able to bang on its output
pin in nominally 10 u-s intervals by treating the PRU's as servo's, taking
position feedback from the machine or the PRU's at 1 ro 5 k/sec intervals.
The main wait there is for the interface boards/capes/shields whatever they
are called for the 'Bone's to stabilize and become available in usable
The best early in the race candidate there seems to be the
"k9smorgasboard", latest revisions at the board house now, with a target
price in the $130 US range, so a nominally $200 USD kit that fits in your
polo shirt pocket is likely to wind up replacing the now discontinued
D-525MW atom board for this duty. Jeff, at xylotex.com is also in the
alpha or beta phase for a somewhat simpler board for the BBBlacks, but
hasn't yet pinned a price tag on it that I can see on his site.
The one disadvantage to me about the Bone is that there doesn't seem to be
an interface to rotating storage, using SSD's instead, and which have a
more limited lifetime compared to the commodity sata drive now in the
pipeline. That will make backups a higher priority for sure.
But this has little to do with the std linux kernel and its horrible
latency's jitter, because despite this one, a 32 bit PAE 3.13.5, that I'm
running ATM being built with rt-preemption, it can still go away for as
long as 30 seconds. And Gkrellm does NOT show a busy core when it does
this so I haven't a clue. It could be taking a break, making yellow snow in
the front yard for all the clues I see. :)
So in my world, we are "living in interesting times" if we have the
sheckles to stay caught up. The technology is moving at visible speeds.
> W. Scott Lockwood III
> AMST Tech (SPI)
> "There are four boxes to be used in defense of liberty: soap, ballot,
> jury, and ammo. Please use in that order." -Ed Howdershelt (Author)
"There are four boxes to be used in defense of liberty:
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-Ed Howdershelt (Author)
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