Pico Cray – Small scale distributed computing | Hacker News Reader

Pico Cray – Small scale distributed computing

extremeelectronics.co.uk

143 pointsecliptik22 comments

It would've been cray-cray to boot them from a single board with extra flash via SWD. Does anyone know how fast the RP2040 SWD clock can be driven on a Pico soldered to a central board? How many DMA channels and PIO state machines would be required to build an even faster remote memory access interface? Can a central 9th RP2040 serve as fast RDMA router for 8 other Pico boards? So many unanswered questions ;-).

One way the RP2040 really punches above his price class is internal bandwidth. It has four AHB-Lite bus masters (two M0+ cores with one read-write port, and the DMA engine with a read and a write port) connected a full crossbar switch. There are six SRAM banks of which the four largest are commonly used word interleaved, but can if you want full control over memory timing and bandwidth you can use the uninterleaved alias instead. Most chips priced around a RP2040 (and the SPI flash to go with it) have only a single small SRAM bank. I've found the DMA engine surprisingly pleasant to work with (easy to understand, no fixed request routing to fight, but still very flexible which can save a lot of interrupts). Whoever designed the combination of the PIO channel I/O coprocessors and this DMA engine had a clever tinkers mind. I have to stop myself from wasting time looking for ways to save CPU cycles with trickery that only makes the code hardware to maintain.

SWD is 25MHz max. IOs on normal ports can reach CPU freq in some cases (so 133-250MHz).

That's the official spec, but given how far the rest of the chip overclocks i would like to check that. The problem is that non of my SWD probes go above 24MHz.

One of the things that I put some though into is that the PIO block is fast and you can build a custom fast interconnect from that (or maybe even Transputer Link) and then build a large distributed memory “supercomputer” from that.

One of the limiting factors is that RP2040 expects to boot from SPI Flash and there is no documented way for external SSRAM (the Synopsys DW IP that is used by RP2040 as SPI flash controller probably even can do that, but its documentation in RP2040 datasheet seems somewhat redacted).

How many Rasperry Pis have the equivalent processing power to an original Cray-1?

(I would not be at all surprised if a single RPi were as powerful as twenty Cray-1s.)

A Cray-1 ran at 80 MHz, and with careful coding could sustain about 2 double-precision floating point operations per cycle - so 160 MFLOPS. Looking at linpack benchmarks, it looks like it's right around a raspberry pi 2 running at 1 GHz (169 DP MFLOPS), and a little worse than a Raspberry Pi 3 at 180 MFLOPS.

The 160MFlOps of Cray-1 is the theoretical maximum of the implementation (as are the whatever FlOp/s numbers quoted by GPU vendors), while the numbers for Linux capable Broadcom SoCs in RaspberryPis are results from LINPACK. The Linpack benchmark is not exactly a representation of what you would compute on a supercomputer, but even this kind of semi-synthetic benchrmark will cause the Cray-1 to score significantly lower than the theoretical 160MFlOp/s.

A Raspberry Pi 4B has 13.5 GFLOPS[0] while a Cray-1 has 160 MFLOPS[1] so you need about 85 Cray-1s :)

Couldn't find any numbers for a Pi Pico.

0: https://web.eece.maine.edu/~vweaver/group/green_machines.htm... 1: https://en.wikipedia.org/wiki/Cray-1

Wikipedia says the Cray-1 was capable of 160 MFLOPS.

As a rule of thumb, modern scalar pipelines can sustain one ALU op per cycle, and you see nearly all Linux-capable CPUs quoted in GHz. So we should expect gigaflops, minimum. And indeed [1] suggests that the Pi 4 is capable of 13.5GFLOP, so about 84 Cray 1's. (The further speedups come from the fact that ARM also has NEON vector instructions, and from multiple cores.)

The Pi Pico, on the other hand, does not have a floating point unit. So it emulates it in software (soft float). The C SDK docs [2] suggest 13.8kHz (!) operation for single-precision add. I'll be generous and suggest that the 2x cores could double this performance. So then, it'd achieve 0.0086% of the Cray-1's performance. Oof.

If you're willing to do integer arithmetic, things look much better for the Pico, of course - it runs at 125MHz and the above scalar rule of thumb applies.

[1]: https://web.eece.maine.edu/~vweaver/group/green_machines.htm...

[2]: https://datasheets.raspberrypi.com/pico/raspberry-pi-pico-c-...

Are you talking about Pis or picos like mentioned in this article.

The original Cray 1 had already had a very powerful vector execution unit and low memory latency relative to the CPU frequency (by todays standards). The RP2040 has even lower memory latency, but only ¼ MiB RAM and no hardware floating point support or SIMD (neither packed-SIMD nor "true" Cray style vectors). At least the BootROM includes hand optimised Soft-FP code and the single-cycle I/O block even includes a memory mapped hardware integer divider and two interpolators per CPU core. These can help a lot with fixed point DSP workloads, but are totally different from what made the Cray 1 special at its time. Todays better MCUs may match certain performance numbers of early supercomputers, but their designs have more differences than similarities. There have been attempts to recreate early Crays in FPGAs, but so little software for them has been preserved (in a way accessible to the public) that it's difficult to judge how good the recreations are and even the later Cray 1 based designs need a really big FPGA to reimplement all relevant pieces at least as fast as the original. Cray didn't just build fast floating point adders and multipliers, but the main memories and interconnects between them to make them useful. Good luck finding a way to attach 100s of interleaved DRAM banks to your FPGA, because your internal block RAM won't be enough as main memory and memory access timings have improved the least over time.

And what's the relative power consumption? :-)

Add in cooling requirements, too.

To be honest, it's not exactly Cray, and not even Cray at all.

Here is the homebrew cycle-accurate Cray-1, and it's a really impressive project:

http://www.chrisfenton.com/homebrew-cray-1a/

But mine was only 1/10-scale!

You're correct this isn't really a Cray other than the vaguely rounded shape, but it is still adorable and looks super fun to program.

About I2C addressing: I've thought about the issue. That solution could fail in theory because of collisions, ie a micro-controller cannot atomically check the assert pin and raise it if low. The project uses 1000ms + rand(0, 5000ms), which sounds big enough.

I've always wondered if a solution based of multi-master I2C support could work: all childs shout a unique ID and the parent answers with their assigned I2C address. Children that do not win the arbitration will detect collisions and stop because of arbitration errors. Unsure how well it could work with many children.

See https://www.i2c-bus.org/multimaster/ and https://www.i2c-bus.org/i2c-primer/clock-generation-stretchi... for introductions to I2C multimaster and arbitration. The spec is of course not open.

I'd be interested in discussing the topic more!

I vaguely remember that Access.BUS (I2C based PC peripheral bus that was kind of a precursor to USB) did address assignment using somewhat similar protocol.

This is adorable!

Always loved the CRAY aesthetic. Saw one in a museum a while back, couldn't get away from it.

Me too – I saw it at the Computer History Museum in Mountain View, CA. Gorgeous machine! The museum was great too, highly recommend a visit to anyone interested in the history of computing.

I think there is a legitimate market for little clusters with blinking lights that sit of your desk.

In the book, The Hunt for Red October, the acoustic signature of the Caterpillar Drive is modeled on a Cray-2 (pretty sure). There's a nice plot segment where the protagonist and the Cray operator make a wager on how long it will take to complete.

In comparison to that model of Cray I'd be curious how the Raspberry's fair. I actually learned linux on a bunch of 486/586 boxen given to me by my neighbor, so powering a Pi2 from my laptop's USB port and getting a full linux distro really impressed me.

Nicely done. I wonder if it can hold a term based BBS server and if so how many users it can serve. But I guess we need to figure out storage first. Is SD card good for pico?

Reply on news.ycombinator.com