From mboxrd@z Thu Jan 1 00:00:00 1970 To: jon@eccincorp.com Cc: Embedded Linux Forum Subject: Re: Interrupt Latency From: Wolfgang Denk Mime-version: 1.0 Content-type: text/plain; charset=ISO-8859-1 In-reply-to: Your message of "Wed, 12 Jun 2002 14:50:32 EDT." <3D0797F8.4030100@eccincorp.com> Date: Wed, 12 Jun 2002 22:16:14 +0200 Message-Id: <20020612201619.CA2DF102F8@denx.denx.de> Sender: owner-linuxppc-embedded@lists.linuxppc.org List-Id: Dear Jon, in message <3D0797F8.4030100@eccincorp.com> you wrote: > > I have an Embedded Planet RPX Classic CLLF_BW31 MPC860 running at 48Mhz with > non-realtime Hard Hat 1.2 with the 2.2.14 kernel. > > In general what kind of interrupt latency can I expect? I am seeing a pretty > consistent 10 us min IRQ2 latency but occassionally see up to 70 us max. I am This is about what you can get on such a CPU. Even with RTAI, running the latency calibration test, you will see average latencies af some 14 us. > running ISR2. First, I was surprised the 10 us min latency, I thought I might > occassionally see a much quicker response. Second, I was surprised to see the What do you expect? This is just a 50 MHz system with tiny caches. This is not the fast number-cruncher you seem to expect... > occassional slow response of 70 us max. It would be nice if we could get the > max under 50 us. Or do you have to go to a real-time kernel? I am looking into Yes, if you need guaranteed response times you have to use a hard RT capable system; my recommendation is RTAI. > and checking the driver code efficiency. Could the non-realtime linux kernel > mask the interrupts for that long? I am going to look into what all linux is Sure it can. Latencies scale more or less with the clock frequency; CPU clock, bus clock and cache size have visible impact. Here are some results we got when running the RTAI latency test on a couple of systems: IBM PowerPC 405GP Rev. D at 198 MHz 16 kB I-Cache 8 kB D-Cache: Interrupt latency approx. 5.9 us MPC855 at 80 MHz / 40 MHz bus clock 4 kB I-Cache 4 kB D-Cache: Interrupt latency approx. 22.0 us MPC860DP at 50 MHz / 50 MHz bus clock 16 kB I-Cache 8 kB D-Cache: Interrupt latency approx. 16.0 us MPC8240 at 247.500 MHz 16 kB I-Cache 16 kB D-Cache: Interrupt latency approx. 4.5 us MPC8260 at 199.999 MHz Interrupt latency approx. 4.0 us PowerPC 750 @ 300 Mhz: Interrupt latency approx. 3.0 us PowerPC 750 at 450 MHz 1 MB L2-Cache Interrupt latency approx. 1.5 us Wolfgang Denk -- Software Engineering: Embedded and Realtime Systems, Embedded Linux Phone: (+49)-8142-4596-87 Fax: (+49)-8142-4596-88 Email: wd@denx.de "I've finally learned what `upward compatible' means. It means we get to keep all our old mistakes." - Dennie van Tassel ** Sent via the linuxppc-embedded mail list. See http://lists.linuxppc.org/