12 meter communication timing

26mar10

Links:
Things to measure
The measurement program
The measurements
Summary

Intro

Timing measurements for the communications with the 12meter controller was done on 26oct10. The setup was:

Controller running "construction program" so the times may not represent those during actual operation.
Linux gettimeofday() call used for time. This gives times to a few micro seconds.
A 100 Mbit connection between computer and controller was used.

The Modbus communications uses the following steps:

send request for read, or request with data registers to write
read back header from controller. When this arrives the controller is usually done with the request
Readback the data registers (if a read). This is very fast since the header and data come back as one Ethernet packet.

Times were measured for the individual steps as well as the total time for an operation. For each time measurement the program kept track of the: min, max, sum , and sum of squares time duration. This allowed the final reporting of: avgTime, minTime, maxTime, and rmsTime.

Things to measure (top)

Ping the controller: about .9 milliseconds
Read 1 32 bit register with a single call
Read 8 32 bit registers with a single call
simulate measuring telescope state:

Status,position, position error,velocity and currents for az,el. Az slave current and status.
Measure state using 3 calls reading: 5words@23683,2words@23693,5words@23703
Measure state using 1 call reading 25 words@23683

Write 1 register with a single call.

The measurement program: (top)

The test program timed the following aspects of the communications:

To Read registers:

start TimeReadTotal

Write LowLevel:

startTime Send Request

send request to controller

stoptime Send Request
Note that this comes back when the send is complete in the OS, not when the controller is done.

Read Back lowLevel Header

startTime: read header reply
read header plus byteLength or exception code
StopTime: read header reply

Read Back lowLevel data:

startTime: read data registers
read data registers
stopTime: read data registers.
Note that this is very fast since the hdr/data came back as one block from the controller.

StopTimeReadTotal

Write Registers:

start TimeWriteTotal

Write LowLevel:

startTime Send Request

send request to controller along with the data registers to write

stoptime Send Request
Note that this comes back when the send is complete in the OS, not when the controller is done.

Read Back lowLevel Header

startTime: read header reply
Readback hdr sent (with optional exception code)
StopTime: read header reply

Read Back lowLevel regAdr,numReg:

startTime: read adr,numreg
read back adr,numreg sent
stopTime: read adr,numReg
Note that this is very fast since the hdr/data came back as one block from the controller.

StopTimeWriteTotal

The measurements: (top)

The first table shows the total time for each measurement.
the 2nd table shows the breakdown of times within each measurement
All times are milliseconds.

Total times for measurement
Description	avg	min	max	rms	numTrys
Read 1 reg per call	8.03	6.09	9.62	0.22	867
Read 8 regs per call	12.41	10.49	14.63	0.60	100
Read State: 12 regs with 3 calls(5reg,2reg,5reg)	30.17	28.68	34.04	1.05	100
Read State: 1 call, 25 regs	21.50	19.02	25.52	1.60	100
Write 1 reg per call	8.39	7.51	10.95	.54	100

TABLE SHOWING BREAKDOWN OF TIMING
READ 1 Register at a time. Cycle over all addresses
description	avg	min	max	rms	NumTrys
write hdr	0.01	0.01	0.02	0	867
read back hdr	8.01	6.08	9.59	0.22	867
readBack 1 reg	0.00	0.00	0.02	0.00	867
Total ReadTime 1 reg	8.03	6.09	9.62	0.22	867

Read 8 Registers at a time (23403-23410)
write hdr	0.01	0.01	0.02	0.	100
read back hdr	12.39	10.47	14.60	.69	100
readBack 8 reg	0.01	0.0	0.02	0.	100
Total ReadTime 8 reg	12.41	10.49	14.63	.60	100

Read Status,position,poserror,velocity,currents for azMaster,slave,Elevation 12 words using 3 reads: 5words@23683,2words@23693,5words@23703
write hdr	0.01	0.01	0.02	0.00	300
read back hdr	10.04	6.69	14.92	0.96	300
readBack 2 or 5 reg	0.01	0.00	0.02	0.00	300
Total ReadTime 12 words	30.17	28.68	34.04	1.05	100

Read Status,position,poserror,velocity,currents for azMaster,slave,Elevation 25 words 1 read: 25 words@23683
write hdr	0.01	0.01	0.02	0.00	100
read back hdr	21.48	18.99	25.49	1.59	100
readBack 25 reg	0.01	0.00	0.01	0.00	100
Total ReadTime 25 words	21.50	19.02	25.52	1.60	100

write single register @23404
write hdr + register	0.01	0.01	0.01	0.	100
read back hdr	8.37	7.49	10.92	.54	100
Total writeTime 1 word	8.39	7.51	10.95	.54	100

Summary:(top)

The controller replies with the header and returned data in a single Ethernet packet.
Reading multiple registers with a single call if much faster than reading 1 register per call

Time to read 1 register: 8 ms.

Time to read 8 registers: 12.4 ms.

Reading state using 3 reads is 50 % slower than reading state via 1 25 word read.
Access to the controller can be sped up by changing the addresses of commonly accessed locations to be contiguous.Things that could benefit from this are:

status words, state info read each second.
loading tracking data could be done in a single write.

processing: svn/aosoft/p12m/testMultiReg.c, testRdSeg.c,testWrite.c,dump_adr.c

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