Dan Malek <dan at netx4.com> writes: > Graham Stoney wrote: > > .... Can anyone give me a rough idea of the maximum latency in user > >space I could expect on an 860T based system with multiple threads, > >where only one thread has SCHED_RR realtime scheduling priority? > > > You can't answer this question due to the design and implementation of > the kernel. Most "real-time" systems can't either once you throw > protocol stacks, file systems, disk drives, and other external devices > or distributed services into the feature mix. > > I have successfully written applications that required sub-millisecond > latency. You just have to be careful about other applications in the > system, and remember that you can't preempt the kernel.
Ingo molnars lowlatency patch (http://www.redhat.com/~mingo/lowlatency-2.2.13-A1) (adapted to PPC ofcourse) in combination with POSIX realtime scheduling can actually do great things for you. (Not hard realtime ofcourse but very good "normal-case" latency even under high load) However, if you use a serial console - don't even think about doing anything remotely realtime without the following patch. Without it the 8xx serial driver can busy-wait up to 25ms(!!!) (assuming 9600 bits/s) when it prints a newline. Index: n_tty.c =================================================================== RCS file: /swb/cvs_repository/linux/drivers/char/n_tty.c,v retrieving revision 1.1.1.1 diff -u -r1.1.1.1 n_tty.c --- n_tty.c 1999/07/07 13:57:03 1.1.1.1 +++ n_tty.c 1999/11/30 20:17:16 @@ -144,11 +144,13 @@ if (O_ONLCR(tty)) { if (space < 2) return -1; - tty->driver.put_char(tty, '\r'); + tty->driver.write(tty, 0, "\r\n", 2); tty->column = 0; + } else { + tty->driver.put_char(tty, c); } tty->canon_column = tty->column; - break; + return 0; case '\r': if (O_ONOCR(tty) && tty->column == 0) return 0; //Marcus -- -------------------------------+------------------------------------ Marcus Sundberg | http://www.stacken.kth.se/~mackan/ Royal Institute of Technology | Phone: +46 707 295404 Stockholm, Sweden | E-Mail: mackan at stacken.kth.se ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/
