[CuriousCosmic]

I definitely agree that a PCI-E card is preferable. Hell even if you have it in CPU, you probably want it sat on the PCI-E bus anyways so it can P2P DMA with other hardware.

Also (not disagreeing but I'm curious), last time I checked FPGAs could pull off some level of partial reconfiguration in the millisecond and sub millisecond ranges. I may be a bit off on these times but I saw them in a research paper a few years back. What types of speed would be necessary for CPUs to actually be able to benefit from a small FPGA onboard (rather than on an expansion card) with all the context switching.

[akiselev]

High end FPGAs are theoretically capable of millisecond fast partial reconfigurations but doing so requires making a lot of tradeoffs that just highlight the impedance mismatch between the generic nature of CPUs and the purgatory that is FPGAs. The more of the FPGA you want to reconfigure, the longer it takes (stop the world, depending on which parts its touches) and unless the reconfigured portion is limited to a standard bus, the reconfiguration won't work (or you have to reconfigure more of the design to deal with different interfaces, timings, etc. blowing up reconfiguration time and defeating the purpose). All of the bitstreams have to be compiled ahead of time as well.

Unless latency is so critical that the speed of light is the limiting factor, partial reconfiguration just replaces PCIe with a much harder to work with AXI interconnect (or similar, but it always end up being AXI...).