Messages in this thread | | | Subject | Re: [PATCH v1 3/3] drivers/tty: increase priority for tty_buffer_worker | From | Oleksij Rempel <> | Date | Mon, 28 Jan 2019 10:22:02 +0100 |
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On 28.01.19 09:23, Greg Kroah-Hartman wrote: > On Mon, Jan 28, 2019 at 09:05:30AM +0100, Oleksij Rempel wrote: >> >> >> On 10.01.19 17:30, Greg Kroah-Hartman wrote: >>> On Thu, Jan 10, 2019 at 04:19:53PM +0100, Oleksij Rempel wrote: >>>>> My gut feel is that if somebody still cares deeply about serial line >>>>> latency, they should look at trying to see if they can do some of the >>>>> work directly without the bounce to the workqueue. We use workqueues >>>>> for a reason, but it's possible that some of it could be avoided at >>>>> least in special cases... And yours sounds like a special case. >>>> >>>> It is for industrial low latency RS-422 based application. The loopback >>>> test is just easy way to test/reproduce it without additional hardware. >>>> >>>> What is good, mainlineable way to implement it? >>> >>> What is the real problem your systems are having? Are they serial-port >>> limited? Is latency a big issue? Trying to tune for a fake workload >>> isn't the best way to solve anything :) >> >> The system in question is a high power laser cutter with live image-based inspection >> and adjustment of the cutting process. In this setup the RS422 interface is used to >> control parameters of the laser cutting unit in a tie control loop with the camera. >> This loops needs to operate at 1000 Hz. >> >> The xy-stage moves with a speed of approx. 60m/min, i.e. within 1ms it >> moves about 1mm. For a high precision control process a jitter of ± 500 us (+/- 0.5mm) >> is unacceptable. > > Are you using the rt kernel patch for this type of thing? That should > bound your jitter at a much more deterministic level.
Yes, I tested it with different linux-rt version with mostly similar results: kernel 4.8.15-rt10+ latency histogram: 0 ... < 250 usec : 1933104 transmissions 250 ... < 500 usec : 21339 transmissions 500 ... < 750 usec : 8952 transmissions 750 ... < 1000 usec : 6226 transmissions 1000 ... < 1500 usec : 7688 transmissions 1500 ... < 2000 usec : 5236 transmissions 2000 ... < 5000 usec : 11724 transmissions 5000 ... < 10000 usec : 3588 transmissions 10000 ... < 50000 usec : 2123 transmissions 50000 ... < 1000000 usec : 20 transmissions >= 1000000 usec : 0 transmissions
kernel 4.9.0-rt1+ latency histogram: 0 ... < 250 usec : 1950222 transmissions 250 ... < 500 usec : 15041 transmissions 500 ... < 750 usec : 5968 transmissions 750 ... < 1000 usec : 4437 transmissions 1000 ... < 1500 usec : 6022 transmissions 1500 ... < 2000 usec : 4185 transmissions 2000 ... < 5000 usec : 9864 transmissions 5000 ... < 10000 usec : 2773 transmissions 10000 ... < 50000 usec : 1462 transmissions 50000 ... < 1000000 usec : 26 transmissions >= 1000000 usec : 0 transmissions
4.19.10-rt8 latency histogram: 0 ... < 250 usec : 1906861 transmissions 250 ... < 500 usec : 35271 transmissions 500 ... < 750 usec : 13103 transmissions 750 ... < 1000 usec : 9084 transmissions 1000 ... < 1500 usec : 9434 transmissions 1500 ... < 2000 usec : 5644 transmissions 2000 ... < 5000 usec : 12737 transmissions 5000 ... < 10000 usec : 4511 transmissions 10000 ... < 50000 usec : 3201 transmissions 50000 ... < 1000000 usec : 154 transmissions >= 1000000 usec : 0 transmissions
without extra CPU load the result on kernel 4.19.10-rt8 will be: latency histogram: 0 ... < 250 usec : 1999992 transmissions 250 ... < 500 usec : 8 transmissions 500 ... < 750 usec : 0 transmissions 750 ... < 1000 usec : 0 transmissions 1000 ... < 1500 usec : 0 transmissions 1500 ... < 2000 usec : 0 transmissions 2000 ... < 5000 usec : 0 transmissions 5000 ... < 10000 usec : 0 transmissions 10000 ... < 50000 usec : 0 transmissions 50000 ... < 1000000 usec : 0 transmissions >= 1000000 usec : 0 transmissions =============================================================
test results with same load and replaced kworker with kthread and assigned an RT priority
min latency: 0 sec : 75 usec max latency: 0 sec : 125 usec average latency: 81 usec latency measure cycles overall: 79000000 latency histogram: 0 ... < 250 usec : 79000000 transmissions 250 ... < 500 usec : 0 transmissions 500 ... < 750 usec : 0 transmissions 750 ... < 1000 usec : 0 transmissions 1000 ... < 1500 usec : 0 transmissions 1500 ... < 2000 usec : 0 transmissions 2000 ... < 5000 usec : 0 transmissions 5000 ... < 10000 usec : 0 transmissions 10000 ... < 50000 usec : 0 transmissions 50000 ... < 1000000 usec : 0 transmissions >= 1000000 usec : 0 transmissions
Kind regards, Oleksij Rempel
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