RE: [Xen-devel] ac_timer: time to say goodbye?

["Magenheimer, Dan (HP Labs Fort Collins)" <dan.magenheimer@xxxxxx>]

> What's heavyweight about it? We can maybe slim down the interface a 
> little, but we need a mechanism for managing at least an 
> alarm timeout 
> per vcpu. I don't imagine anyhting much simpler than ac_timer would 
> really fit the bill.
> 
>   -- Keir
>
> Heavy weight? It's a basic heap (priority queue) implementation. 
> Seems to me like a perfectly sensible thing to have in a 
> hypervisor, and
> its used extensively by all the schedulers.
> 
> Thanks,
> Ian

I can't say I fully understand the code and usage, so I may
be missing something, but...

Is there ever any more than one or two elements in the queue?
What is the total set of functions called by the queue?

> its used extensively by all the schedulers.

Try 'grep -r ac_timer' and see.

I suspect that the whole functionality of it can be replaced
with a couple of time variables that are checked and
manipulated in the timer interrupt code and a single
scheduler/timer routine in the generic scheduler.

The queueing mechanism is nice if there are a lot of uses,
but confusing (and IMHO heavyweight) if not.  I suspect
the generalized mechanism was good when there was more
functionality in Xen itself but as more and more migrates
to domain0, if the remaining usage could be replaced by
a couple variables, perhaps it should.

Not urgent, but if this is the right direction, we shouldn't
be layering more code on top of it (thus the suggestion of
deprecating it).

Dan

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