Documentation: Kernel <-> Host Attribute Synchronisation/writeback

[pathfinder49]

Question

Category: Attribute Writeback

Description

The kernel-host attribute synchronisation behaviour is not apparent to new users from reading the documentation.

The code below is intended to demonstrate the current behaviour:

from artiq.experiment import *

class AttributeTest(EnvExperiment):
    def build(self):
        self.setattr_device("core")

    def prepare(self):
        self.a = "A"

    @kernel
    def run(self):
        print("krun")
        print(self.a)
        self.rpc() 
        print("krun after rpc")
        print(self.a)
        self.a = "A_2"
        print(self.a)
        self.rpc()

    def rpc(self):
        print("rpc")
        print(self.a)
        self.a = "A_1"
        print("rpc modified")
        print(self.a)

This prints:

krun
A
rpc
A
rpc modified
A_1
krun after rpc
A
A_2
rpc
A_1
rpc modified
A_1

I've been aware of this behaviour for some time. However, I can't quite decide if it is intended behaviour or a bug. It also raises the question of why kernel_invariants need to be explicitly specified (given that attribute changes outside the compiler are not synchronised to the core device).

[dnadlinger]

Attribute writeback happens only when the kernel terminates, not before/after RPCs.

I haven't looked at the docs for this in a long time, maybe they could be improved.

kernel_invariants is for modifications done by the kernel, where assuming values to be constant enables compiler optimisations. (Currently, this isn't taken advantage of nearly as well as it could.)