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by ChuckMcM 4262 days ago
Actually the Raspberry Pi does not run circles around it because it has to do so much of its work through the USB bus. The Pi makes a good case study for system architecture in that regard. The trick is to compare channel bandwidth, memory, I/O, and cache to see see how those parts interact.
1 comments
bodyfour 4262 days ago
Sure, let's compare. I don't have every number at hand, but:

I/O: * SS10 -- Fast SCSI: 80 Mbit/sec theoretical max * Rpi -- USB2 480 Mbit/sec. Now there are certainly complaints about poor USB performance, but it's not so bad that the SS10 would beat it

Cache: * SS10 -- 32K in the base model (although later ones had more) * Rpi -- 256K

RAM: * SS10 -- depends on configuration; most machines of the era tended to have 32MB or so. I actually had an SS10 with an exotic 128MB, but half of the ram was actually on an SBus card. * Rpi -- 256MB in the lowest configuration you can buy. True, that is shared with the GPU whereas the SS10's CG3 card had its own memory if I remember right.

You are correct that the Rpi is bus and I/O crippled compared to most modern hardware. That's why I chose it as an example: even the cheapest hardware you can buy today is so much faster than a SS10. It's really astounding to think about.

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ChuckMcM 4262 days ago
And that is why it is such a great tool for learning. Lets start here:

   > I/O: * SS10 -- Fast SCSI: 80 Mbit/sec theoretical max 
   > * Rpi -- USB2 480 Mbit/sec. Now there are certainly 
   > complaints about poor USB performance, but it's not 
   > so bad that the SS10 would beat it
The SBus[1] is slower than High Speed USB[2], and yet the amount of code it takes to get 1 megabyte of data from a USB peripheral into the memory of a CPU is greater. USB is polled, the Host Controller has to continually check to see if there is something out there, SBus is not allowing it to initiate transfers from the peripheral. Further things like key presses compete with network packets for attention from the root HUB. While transactions on the Sbus(es) can happen in parallel.

Then there is the SDIO interface for the RPi's flash disk, which also competes with USB for I/O cycles on the ARM AHB bus. So at the end of the day some things are really faster (like graphics) and some things slower (like I/O), and the whole package feels remarkably similar. Can't argue on the price though, you still can't get a working SS10 for $35 as far as I can find.

[1] SBus specification -- http://bitsavers.informatik.uni-stuttgart.de/pdf/sun/sparc/8...

[2] My go to USB spec site -- http://www.beyondlogic.org/usbnutshell/usb1.shtml

[3] Summarized SS10 information -- http://www.obsolyte.com/sun_ss10/

[4] Gateway to the simplified SDIO specs -- https://www.sdcard.org/developers/overview/sdio/sdio_spec/

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kabdib 4262 days ago
Usable USB2 bandwidth is about 40 MBytes/sec (you can squeeze out more, but you're talking custom devices and carefully crafted protocols).

80 MBit/sec SCSI is 8 bits wide, with pretty good utilization (depends on the drive), so it's going to do pretty well in comparison. I don't know if 1992-era drives could do (say) 100 MBytes/sec, maybe on the outer rims?

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bodyfour 4262 days ago
> 80 MBit/sec SCSI is 8 bits wide

No, I converted both of the speeds to MBit to make them easier to compare. The SCSI on a SS10 was 10MB/sec, so 80Mb/sec.

The bus utilization on SCSI was high, and the USB2 on a Rpi is slow but not nearly enough to make up for a 6x gap in raw bus speed.

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