Where is the difference - 10 times X1 vs 1 times X10

Danal

Somewhere up there, DC42 pointed out it will always be possible to generate GCode that exceeds any given planners ability to read/parse/plan/execute. My example would be: Keep lowering the distance moved per command, and the result is an arbitrary number of lines needed to reach the same endpoint of the move. The 300 X1 in a row is probably an example of this.
Therefore this whole discussion is only relevant if it affects actual (useful) gcode as generated by slicers or CAM or similar. (Dave said this as well)

Those two points combined seem to have triggered lots of discussion about optimizing G-code between the slicer and the actual execution in the firmware. Where/when/how much to optimize... lots of good discussion. At the same time, point 2 makes me really wonder if that optimization is needed?

arhi

@DocTrucker said in Where is the difference - 10 times X1 vs 1 times X10:

My understanding of real time is that something needs to be done at a specific time, within a tight tollerance, not early or late.

Well, "real time" usually, in IT industry, means that the time is known. For e.g. MySQL Cluster Carrier Grade Edition (MCCGE) is real-time rdbms. The query will either finish in known time or it will fail (with temporary error and you can decide to retry or move forward depending on what you need at that moment). The major part is that you know/can calculate the time and not have that time be dependent on stuff you cannot control (e.g. mjpegstreamer running in parallel). While most ppl when they talk assume real-time means fast, the time does not need to be short at all, it just need to be known for the system to be real-time.

With regards to klipper it works more less like LinuxCNC, so like LinuxCNC uses HAL to send stepping info to the drivers connected on some bus (pci, usb, ethernet), klipper sends similar info to it's drivers connected on usb. Does the klipper itself require RT or not I could not say but I tend to trust that @dc42 knows what's he's talking about as duet3 and CAN drivers are IMHO same thing. He sends the stepping instructions to CAN drivers just like klipper send it trough usb or linuxCNC via HAL trough any of the supported busses.

arhi

@Danal said in Where is the difference - 10 times X1 vs 1 times X10:

At the same time, point 2 makes me really wonder if that optimization is needed?

As I mentioned, I did the test, because of a lot of disturbance with octoprint+marlin and curves where running gcode through octoprint will create blobs and other artifacts on the print while running from sdcard would print ok. The major problem was found to be the small serial port buffer on the marlin so it's unable to receive enough statements to run uninterrupted. While increasing the buffer solves 90% of the issues, 10% still exist and those were more/less tracked to both SBC and marlin CPU not being able to calculate CRC fast enough to have enough G-Code pass trough and finally if the movements are even shorther the issue with parser & planner on the 8bit mcu unable to execute code. All this with real world sames, STL generated by fusion or solidworks on "high" settings, sliced with "printer profile" in cura or prusa or s3d..

I never had any of those problems with smoothieware so when I got my hands on the duet2ethernet it was the first thing I tested and it passed with flying colors, the stuff marlin could not print from SD card duet printed through USB.

So I'd say - real world problems don't exist with RRF.

BUT!

The work I read on that I mentioned few times, that's ATTM just a plugin for octoprint but that actually recodes the G-Code optimizing it by matching curves on top of the existing code reducing both code size and code complexity is IMHO "next step" in home printers evolution. It can be run as stand alone optimize if=lalala.gcode of=lalala-optimized.gcode for non octoprint users (I prefer it that way even for octoprint so it don't do stuff on the fly) and "tomorrow" it can be a plugin for cura/ps/ideamaker/s3d...

Now, I heard KISS beta version have this already in the slicer, dunno free or commercial version I never used that slicer so can't say, but if anyone is, check beta version, advanced options, curve matching or something like that ... there are branches of slic3r with curve matching too .. so probably in the near future .. maybe even f360 ..

But it's def. not something firmware needs to do. Nice thing to have externally but firmware just need to support and properly handle G2 and G3

Danal

Klipper's central tasks do not require any RTOS.

Klipper makes sure the Pi based portions of the system "stay ahead" of the "stepper driver boards". I don't know by how much it attempts to stay ahead. This was specifically designed so that the stepper boards are 'real time' pulse generators and the 'central' (e.g. Pi) is absolutely NOT 'real time', so that the central parts are able to tolerate the variances in execution that come from being a task under Linux.

I seem to remember Dave saying that Duet 3 (with or without Pi) sends commands on CAN about 1/2 second before they are needed by the expansion or tool board. Similar idea. It is true that the 6HC is running an RTOS, but the central-to-can pieces are 'in advance' so that the actual pulses can happen 'real time', later, as synchronized by tick count.

Conceptually similar; different in detail.

dc42

@Danal said in Where is the difference - 10 times X1 vs 1 times X10:

Klipper's central tasks do not require any RTOS.

Every task that has a to meet a deadline to maintain the expected results/performance is a real time task. Step pulse generation is hard real-time, so hard that it is normally done by an ISR, not an RTOS task. Planning is a softer real-time task, because occasional underruns in the movement queue can be tolerated. Reading and parsing GCode commands from file is softer still.

From an engineering POV, Klipper should be using a RTOS for the planner. It gets away without using one because there is usually enough CPU power to keep up, especially when using a multicore Pi and the Pi is not being used for anything else (e.g. slicing).

BTW on Duet 3 we pre-parse the GCode file on the Pi, so the GCodes fed to the Duet are already partly processed.

It will be interesting to see how Klipper and Duet 3 compare when printing "difficult" GCode files and running other tasks such as slicing, USB webcam etc. on the Pi at the same time.

arhi

@dc42 said in Where is the difference - 10 times X1 vs 1 times X10:

It will be interesting to see how Klipper and Duet 3 compare when printing "difficult" GCode files and running other tasks such as slicing, USB webcam etc. on the Pi at the same time.

klipper users normally run octoprint with ton of plugins (some quite heavy on the cpu) in parallel, most run mpegstreamer there with usb or rpi camera but I know more tha one running 3-4 usb cameras there with as many streamers in parallel .. noone reported a problem. Have not tried myself but follow klipper groups closely as I was planing to. Also I use octoprint on my non-duet printers so have some experience on the load plugins can put on the sbc. No clue how they do it but they do. I guess they just have a large queue, you can delay step starting 10 seconds easy and have 10 seconds buffer for "glitches" and "problems" with cpu alotment

gtj0

@dc42 Can't speak for a Pi specifically, but my Jetson Nano runs a web cam, @Danal 's DuetLapse, ustreamer, and DueUI in a Chrome browser that actually shows the web cam stream. With all that running it keeps the 4 core Nano at about 40% CPU. With that going on, I can print Covid-19 masks (which are all curves) at 100mm/s without any issue at all.

Danal

@dc42 said in Where is the difference - 10 times X1 vs 1 times X10:

Every task that has a to meet a deadline to maintain the expected results/performance is a real time task. Step pulse generation is hard real-time, so hard that it is normally done by an ISR, not an RTOS task. Planning is a softer real-time task, because occasional underruns in the movement queue can be tolerated. Reading and parsing GCode commands from file is softer still.

From an engineering POV, Klipper should be using a RTOS for the planner. It gets away without using one because there is usually enough CPU power to keep up, especially when using a multicore Pi and the Pi is not being used for anything else (e.g. slicing).

Good clarity. Missing a deadline, even at the medium soft levels, ruins a print. Running "far enough in advance to usually get away with it" is not the same.

bot

@dc42 Well, well, well... seems Klipper users have encountered MCU bottlenecks.

Not trying to pick on anyone, just found a relevant issue come to light and I thought it would be nice to tie it all in.

arhi

@bot looking that log file he's not going over 40%... and is using rather conservative settings ( 1/16 on ramps with 80mm/sec print and 130mm/sec move) .. I guess this would be seen a lot if it's a common problem (especially as a whole lot is using 1/32 and 1/64) ... something smells bad there

bot

It seems that the new version of Klipper has potentially introduced some "MCU-stopping" bugs. It could be related to their very interesting "input-shaping" feature.

But yeah, this is on a RAMPS, so I'm really just poking fun. The point is, it happens.