The direct measurement and the measurement recreated by integrating the E-fields data. Using an SMA connector to provide a direct measurement of the edge speed and a waveguide-field probe to sample the fields from the PCB trace, two results are obtained for comparison. Using a Picotest VRTS3 demo board, which has 50 Ohm signals at about 375 ps, this is about the closest measurement we have that is reasonably close. Since this is an unusual measurement, the method should first be validated, performing the measurement on a known switching speed, of similar magnitude. In an earlier post, How Can You Determine the Turns Ratio of a Power Transformer in a Sealed DC-DC Module?, we showed that it is possible to recreate a switching waveform using the E and H fields surrounding the switch. Unfortunately, no such probe exists, and if it did, the PCB trace inductance would still impact the measurement. The probe also needs to have a flat response up to a few GHz and provide a usable dynamic range of more than 100 V. GaN is very low capacitance, so the probe must be near-zero capacitance. With the GaN switching 100 V at an expected bandwidth of above 1 GHz, the selection of the probe is critical. Yep, the null pointer! You can't read from this entire page from user-space anyway :) This was back in 2005, but it's probably the same now unless the CPU state information has grown larger than a page size, in which case they might have changed the implementation.How fast is the world’s fastest GaN switch? It is an interesting question, and maybe is mostly wanted for bragging rights, but the measurement does present some challenges. A context switch is still pretty heavy, while a kernel trap is basically free.Īnswer to at which memory location in user-space, for each process:Īt address zero. This was at relatively short (low-latency) process time slices, though.Īnyway. I found that it could speed-up a Linux machine with lots of heavy processes sharing the same core with about 5%. Implemented support for this magic cache I called CARD (Context-switch Active, Run-time Drowsy) in Linux, and benchmarked rather heavily. When I was at the university, I "invented" (well, I did come up with the idea, knowing that there is plenty of dye in a CPU, but not enough cool if it's constantly powered) a cache that was infinite size although unpowered when unused (only used on context switches i.e.) in the CPU, and implemented this in Simics. It will (likely) have nothing still in the cpu cache from last time it ran, so each memory read will be a cache miss, and needed to be read from RAM. It means storing all processor state (registers, etc) to RAM (at a magic memory location in the user process space actually, guess where!), in practice dirtying all cached memory in the cpu, and reading back the process state for the new process. So, nowadays, a kernel trap is basically just a couple of cpu instructions, hence rather few cycles, compared to tenths or hundreds of thousands when using an interrupt (which is really slow on modern CPU's).Ī context switch between processes is heavy. This is done in the Virtual Dynamic Shared Object (vdso) page in each process (cat /proc/pid/maps to find it) IIRC. Around 2005 (don't remember the kernel version), and after a discussion on the mailing list where someone found that trapping was slower (in absolute measures!) on a high-end xeon processor than on an earlier Pentium II or III (again, my memory), they implemented it with a new cpu instruction sysenter (which had actually existed since Pentium Pro I think). Trapping from userspace to kernelspace used to be done with a processor interrupt earlier. Switching between processes (given you actually switch, not run them in parallel) by an order of oh-my-god.
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