Calling the same Janet function multiple times from C efficiently ?

[janet-lang]

Calling the same Janet function multiple times from C efficiently ?

[bakpakin]

Just an update, I have just pushed some changes to the vm and janet_call that should make janet_call much faster. I was able to make janet_call use the current fiber and not create new fibers.

The changes are in 8a3f512.

[bakpakin]

Side question : Is there a better way to ask such question than opening a "issue" ?

Check out the #janet IRC channel on freenode

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Janet currently does have a bit of overhead going from C to Janet. This will shrink in the future but may never disappear entirely. When calling a Janet function from C, via janet_call, the vm needs to do some bookkeeping for safety reasons.

• Create or reuse a fiber with the function.
• Use setjmp to catch errors so resources aren't be leaked.
• Update global state to point to the new fiber, mark it as a gc root, etc.
• Enter the VM via the funciton run_vm in vm.c

Altogether, this accounts for some overhead. In the future, I plan on adding a pool for fibers so creating
new fibers will be very fast and is often just reusing old fibers. There are other optimizations that I have been considering that would also remove the need for some of these steps, such as having janet_call run on the current fiber and not create a new fiber at all. This can be difficult, however, as we want to prevent any possibility of resource leaks when extending the vm.

For now, I would not worry about the overhead as it should improve, and just stick to janet_call. I think there is a lot of room to improve the overhead without changing the API. There is also the function janet_continue which lets one resume a fiber, and janet_fiber_reset, which lets you take an old fiber and reinitialize it with a new function. These will be much less convenient than janet_call, though.

Other notes:

janet_unwrap_* do not check the type of their arguments. Use

if (janet_checktype(val, JANET_NUMBER)) {
  /* is a number */
} else {
  /* is not a number */
}

to check if val is actually a number. The janet_get* functions check their arguments and provide nice error messages.

[jfcap]

Thanks you for your detailed response.

[honix]

Sorry, I don't like irc as it doesn't save message history. I would better prefer to use issues for this kind of questions.

[bakpakin]

Perhaps a mailing list would be better?

[bakpakin]

I guess issues work for now, but I’m not sure that discussion is what github intended them for. Keeping everything centralized on github is convenient though. Perhaps a chat service like gitter would work too.

[honix]

Gitter seems a good choice.

[bakpakin]

I have started a Gitter channel here, hopefully this is easier than IRC.

[jfcap]

Indeed it's really faster.
My basic benchmark show a ~6X speed improvement !

Now the C version of transform function is 3 times faster then the janet version.
Well done.