Re: Tooltips GC overhead

[David Kastrup]

Nix <address@hidden> writes:

> On 8 Aug 2015, David Kastrup uttered the following:
>
>> Stefan Monnier <address@hidden> writes:
>>
>>>> The cost you're trying to defend against here is a mispredicted branch
>>>
>>> No, I assume the branch would be properly predicted since it would
>>> always take the same path.
>>
>> If cons is encountered with high frequency.  But of course, if it is
>> not, why bother in the first place?
>
> In that case I have no idea what you're trying to defend against. The
> cost of a single conditional and predicted branch is drowned in the
> overhead of the allocation that cons has to do anyway.

Cons cells are pooled, so in a long session the consolidated cost is
that of getting from the free list (cheap) and sweeping back into it
once a mark phase no longer visits the cell.  Which would be the rule
rather than the exception for high-frequency consing.

> I'd be astonished if you could ever see any performance impact
> whatsoever.

Cons cells for Lisp should end up quite cheaper than arbitrary-size
allocations.

-- 
David Kastrup