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SubjectRe: interrupt latency

So you ran the test a bit and got 8 microseconds! Shake in your
shoes or not, your original numbers are way off.
Your calibrate number is most likely way too
high. Again, the problem is worst case versus average. The first
call to "tim" causes instruction cache misses ... The later calls
will be faster. There may be other problems, but I sincerely suggest
you do the whole thing using some macros. It's not hard and avoids
experimentor bias. Also run the tests for at least an hour and
for a week if you are serious.

Anyways, I simply report our measurements. There are other sources of
long delays in PC systems. For example, the measurement test

cli
rdtsc
rdtsc
sti

can show results of 200 microseconds on some P4s. Numbers vary depending
on ethernet cards, IDE interface, chipset, ....
When we test, we try to find worst case. Ping flood is a generally good
source of worst case delays.





On Wed, Aug 28, 2002 at 11:02:57AM -0400, Richard B. Johnson wrote:
> On Wed, 28 Aug 2002 yodaiken@fsmlabs.com wrote:
>
> > On Wed, Aug 28, 2002 at 08:18:25AM -0400, Richard B. Johnson wrote:
> > > No, no, no. There is no such port read that takes 18 microseconds, even
> > > on old '386 machines with real ISR slots. A port read on those took
> > > almost exactly 300 nanoseconds and, in fact, was the limiting factor
> > > for the programmed I/O devices on the ISA bus.
> >
> > Amazing how they can do that with a bus clock that is much slower -)
>
> The ISA bus is not a clocked bus. It is entirely asynchronous. The
> fact that there is a clock on the bus is irrelevant. It is not used
> for any bus-related operations and, in fact, its phase is not guaranteed.
>
> Now, I read your previous post which claimed that what I stated was
> impossible and that I didn't measure anything useful. Now, I note
> with trepidation that you are in some kind of "real-time-Linux" business
> for which I am supposed to be shaking in my shoes. However, you are
> spewing much hype for which I have no countenance.
>
> Here is a re-write that takes your "min and max" rdtsc readings. You will
> note that even the time to write to memory is included in the numbers.
>
> Script started on Wed Aug 28 10:34:10 2002
> # cat usermode.c
> #include <stdio.h>
> #include <unistd.h>
> #include <signal.h>
> #include <asm/io.h>
>
> extern int iopl(int);
> extern long long tim(void);
>
> volatile int run=0;
> void timer(int unused)
> {
> run = 0;
> }
>
> int main()
> {
> unsigned long long ticks_sec;
> unsigned long long calibrate;
> unsigned long long ticks_port;
> unsigned long long worst = 0;
> unsigned long long best = ~0;
> unsigned int count = 0;
> double ns;
> char foo[1];
> (void)iopl(3);
> (void)tim();
> calibrate = tim(); /* Time to make the function call */
> fprintf(stdout, "Wait.....");
> fflush(stdout);
> (void)signal(SIGALRM, timer);
> (void)alarm(1);
> run++;
> (void)tim();
> while(run)
> ;
> ticks_sec = tim() - calibrate;
> for(;;)
> {
> __asm("cli");
> (void)tim();
> foo[0] = inb(0x378); /* Actually put into memory */
> ticks_port = tim() - calibrate;
> __asm("sti");
> if(ticks_port > worst)
> worst = ticks_port;
> if(ticks_port < best)
> best = ticks_port;
> if(!(count++ % 1000000))
> {
> ns = (double)worst / (double)ticks_sec;
> ns *= 1e9; /* Nanoseconds */
> printf("Worse case ticks = %llu\n", worst);
> printf("Worst port read took %f nanoseconds\n", ns);
> printf("CPU ticks/second = %llu\n", ticks_sec);
> printf("Best case ticks = %llu\n", best);
> ns = (double)best / (double)ticks_sec;
> ns *= 1e9; /* Nanoseconds */
> printf("Best port read took %f nanoseconds\n", ns);
> fflush(stdout);
> }
> }
> return 0;
> }
>
> # cat rdtsc.S
> #
> #
> #
>
> .data
> lastl: .long 0
> lasth: .long 0
> .text
> .align 8
> .globl tim
> .type tim@function
>
> #
> # Return the CPU clock difference between successive calls.
> #
> tim: pushl %ebx
> rdtsc
> movl lastl, %ebx # Get last low longword
> movl lasth, %ecx # Get last high longword
> movl %eax, lastl # Save current low longword
> movl %edx, lasth # Save current high longword
> subl %ebx, %eax # Current - last
> sbbl %ecx, %edx # Same with borrow
> popl %ebx
> ret
> .end
>
> # gcc -O2 -o usermode usermode.c rdtsc.S
> # ./usermode
> Wait.....Worse case ticks = 624
> Worst port read took 1572.445620 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 624
> Best port read took 1572.445620 nanoseconds
> Worse case ticks = 1121
> Worst port read took 2824.858237 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1209
> Worst port read took 3046.613389 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1209
> Worst port read took 3046.613389 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1349
> Worst port read took 3399.405676 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1417
> Worst port read took 3570.761929 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1549
> Worst port read took 3903.394656 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1549
> Worst port read took 3903.394656 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 1549
> Worst port read took 3903.394656 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
> Worse case ticks = 3165
> Worst port read took 7975.625621 nanoseconds
> CPU ticks/second = 396834073
> Best case ticks = 529
> Best port read took 1333.050854 nanoseconds
>
> # exit
> exit
>
> Script done on Wed Aug 28 10:35:35 2002
>
>
> The best-case of 529 ticks seems stable, therefore likely what
> an inactive machine will produce this, 1,300 nanoseconds to
> get data from a port into memory.
>
> The worse-case may not have happened yet even though the numbers
> are stable, but I show 8 microseconds, no where near 18 microseconds
> and, if I disconnect my network card so the PCI/Bus wasn't
> continually grabbing everything via Bus Mastering, the best case
> and the worse case ticks are within 400 ticks of each other.
>
> FYI, if you make a real-time system, you must control the Bus Masters
> on the bus, otherwise you can't guarantee anything, and again, that
> is not "latency". It's something else.
>
>
> Cheers,
> Dick Johnson
> Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
> The US military has given us many words, FUBAR, SNAFU, now ENRON.
> Yes, top management were graduates of West Point and Annapolis.

--
---------------------------------------------------------
Victor Yodaiken
Finite State Machine Labs: The RTLinux Company.
www.fsmlabs.com www.rtlinux.com

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