series of switchpoints or better

[Chris Seaton]

Hi Jochen,

I’m not an expert on the implementation of switch points, but my understanding is that they don’t appear in the dynamically compiled machine code at all. They use the safe point mechanism of the VM (the same thing that does the stop-the-world in the garbage collectors) for which polling instructions are already there anyway.

http://chrisseaton.com/rubytruffle/icooolps15-safepoints/safepoints.pdf <http://chrisseaton.com/rubytruffle/icooolps15-safepoints/safepoints.pdf>

See figure 6 (no significant difference in runtime with switch points there or not), and figure 9 (machine code contains no trace of them). So switch points aren’t just fast - they don’t take any time at all. (Ignore the references to Truffle if you aren’t using that.)

I don’t think any of this would change no matter how many of them you have.

I’m sure they do have an impact on interpreter performance, of course, where they can’t be optimised away.

I suppose it could conceivably be the case that a great many switch points may start to upset the compiler in terms of things like inlining budgets? I’m not sure, but seems unlikely.

Chris

> On 5 Oct 2016, at 13:37, Jochen Theodorou <blackdrag at gmx.org> wrote:
> 
> Hi all,
> 
> I am constructing a new meta class system for Groovy (ok, I say that for several years already, but bear with me) and I was wondering about the actual performance of switchpoints.
> 
> In my current scenario I would need a way to say a certain group of meta classes got updated and the method for this callsite needs potentially be reselected.
> 
> So if I have class A, class B and then I have a meta class for Object and one for A.
> 
> If the meta class for A is changed, all handles operating on instances of A may have to reselect. the handles for B and Object need not to be affected. If the meta class for Object changes, I need to invalidate all the handles for A, B and Object.
> 
> Doing this with switchpoints means probably one switchpoint per metaclass and a small number of meta classes per class (in total 3 in my example). This would mean my MethodHandle would have to get through a bunch of switchpoints, before it can do the actual method invocation. And while switchpoints might be fast it does not sound good to me.
> 
> Or I can do one switchpoint for all methodhandles in the system, which makes me wonder if after a meta class change the callsite ever gets Jitted again. The later performance penalty is actually also not very attractive to me.
> 
> So what is the way to go here? Or is there an even better way?
> 
> bye Jochen
> 
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