The interesting thing about all this is the DPC latency vs. performance is a binary situation. It works until it doesn't. You don't observe a gradual degradation of performance.
I think what you are dealing with when you are within the "acceptable range" of DPC latency is headroom to a certain degree. I have to agree with Neal is if it works and you are happy, there really isn't a reason to muck with it or spend $200 bucks to get a 15% reduction in DPC latency if your average is below 100 us. Does lowering the DPC latency numbers give you better "protection" when running other programs? Absolutely not. Example, I have a Core 2 Duo machine whose average DPC latency is ~50us. I can load up several other programs, some ham related, others not and now my average latency numbers are ~110 us with "spikes" up to 250 us. Still VERY respectable. Now, I can load this one little OBNOXIOUS program and WHAM! PowerSDR locks up repeatedly and reproducibly. That program is MS Outlook using the HTTP/RPC transport. Will a 20us DPC latency machine stop this from happening? No, it will not because Outlook will still generate 4000+ us DPC latencies. The only way to reduce DPC latencies that are this high is to stop using programs that cause you grief. Now, if you average system DPC latency is on the order of 1000 us, then by all means getting a PC that has a MUCH lower average DPC latency will allow you to run things on your PC that you might otherwise not be able to do. The bottom line is usability. If PowerSDR is working with little to no (my preference) freezes, then it really doesn't matter. BTW, not all PowerSDR "freezes" are DPC related. If you have low DPC latency and PowerSDR is freezing up, you a RFI issue. Either near field RF exposure or more likely common mode RF currents are getting back into the computer via the Firewire cable. Makes sure the computer and SDR hardware are grounded well. -Tim -----Original Message----- From: flexradio-boun...@flex-radio.biz [mailto:flexradio-boun...@flex-radio.biz] On Behalf Of Ross Stenberg Sent: Sunday, February 14, 2010 9:12 AM To: flexradio@flex-radio.biz Subject: Re: [Flexradio] Things I have learned after 8 months selling Flex-readycomputers! Neil, thank you for sharing your knowledge with us. I see that your goal is avg 20us DPCs. What is your take on performance hits due to higher DPCs? I believe the Flex position is that performance should be acceptable if DPCs are no more that 1000us as shown on the their driver DPC latency checker. Does it really make a difference if ones DPC is 138us or 543us? 73 Ross K9COX _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/ _______________________________________________ FlexRadio Systems Mailing List FlexRadio@flex-radio.biz http://mail.flex-radio.biz/mailman/listinfo/flexradio_flex-radio.biz Archives: http://www.mail-archive.com/flexradio%40flex-radio.biz/ Knowledge Base: http://kc.flex-radio.com/ Homepage: http://www.flex-radio.com/
