Synchronous processors

Imagine a processor with no interrupts. We can do a lot better and get rid of most exceptions (e.g. system calls, page faults etc.), most peripheral devices/buses, and even cache misses, but let’s start with interrupts. Modern micro-processors are bloated with circuitry that is designed to allow the precious CPU to switch back and forth between streams of code because cpu cycles were the scarcest resource. That was a great idea in the 1950s. But interrupts are expensive in compute time circuit complexity, and chip area. Interrupts  take microseconds to start processing – which is an eternity on a Ghz processor.  And they solve a problem that does not exist anymore: we have a lot of processor cores. In fact, one problem faced by computer architects is that it’s not easy to exploit parallel processing if you keep thinking in terms of 1950s computer architecture.

Suppose you have 32 cores and you make one or two of them poll I/O devices and the other 30 never get I/O interrupts, never run interrupt handling code, never have to save and restore state or context switch due to I/O interrupts.  What they do is run application code to completion or for a specified number of cycles or clock ticks. The cores reserved for operating system use manage all devices and even they don’t need interrupts because they can run a real-time OS that polls devices.

Suppose we have 100 devices and flags on device registers to indicate whether the device needs attention. We’d probably want devices to be able to do DMA (but see below). The polling software might just cycle through in arbitrary order or in order of priority or to optimize some function.

For an x86 type processor: throw away the IOAPIC,  IOAPIC bus, the APIC on each core, the logic for unwinding the pipeline on an interrupt, the processor interrupt level logic. And then contemplate the layers of interrupt virtualization in modern processors. We could have a simple cycle timer switch on each core so that after the timer expires there an interrupt-like jump to a function to see what to do next. That jump would be perfectly synchronous since predicting the next jump can be done with 100% accuracy (or nearly 100%).

Once I/O interrupts go away, radical redesign of peripheral devices starts to seem sensible. Instead of putting an embedded computer on an Ethernet adapter card or chip, let’s use one of our I/O cores which already can access memory and knows how to get data to the right processes or OS components. Current OS offload ends up recreating the same bottlenecks on the adapter, but the OS cores don’t have the same problems.  The adapter can be simplified and optimized for speed and error correction.  A hardware fifo of addresses for TX and RX queues perhaps: header, payload start, payload end, condition code. The OS core software has to keep TX queue from sitting idle and the RX queue from overflowing. RDMA is reasonably simple in this environment.

Do we need system calls with all the complex context and permission level change? I don’t see why: a system call could queue a request for the OS and then either call code to voluntarily switch or keep going – which might allow the same application to respond to the result on a second core or an event model or … .  For some application loads we’d maybe need to steal a few more cores for OS functions, but the OS is now much simplified without all these weird asynchronous operations.

Getting rid of page faults/memory faults and arithmetic error exceptions seems like it needs a slightly bigger change to programming models, but not all that radical.    More on that later


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  1. Pingback: Processor architecture – keeping simple

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