5 ms·

well if you're making a synchronous call you have to wait for the response which is likely as bad as a pipe flush (or worse, because you likely flood the pipe w

well if you're making a synchronous call you have to wait for the response which is likely as bad as a pipe flush (or worse, because you likely flood the pipe with a tight loop waiting for the response, or a context switch to do something else while you wait) Also note that stealing a cache line can be very expensive, if the CPUs are both SMT with each other it's in the same L1, almost 0 cost, if they are on the same die it will be a few (4-5?) clocks across the L2/cache coherency fabric but if they are on separate chiplets connected via a memory controller with L3/L4 in it then it's 4 chip boundary crossings - an order or 2 in magnitude in cost

kragen 5y ago
All that makes sense to me. So for high performance collaboration across security boundaries needs to be either very rare or nonblocking? Multithreading within a security boundary is one way to "synchronously wait" without incurring a giant context-switch cost (SMT or Tera-style or Padauk FPPA-style; do GPUs do this too, at larger-than-warp granularity?). Event loops are a variant on this, and io_uring seems to think that's the future. But the GreenArrays approach is to decide that the limiting resource is nanojoules dissipated, not transistors, so just idle some transistors in a synchronous wait. Not sure if that'll ever go mainstream, but it'd fit well with the trend to greater heterogeneity.