Chris Angelico wrote:
> We have people who believe that a bit
> mask will slow things down, others who claim that improved cache
> locality will speed things up, and Mark asks us to please justify our
> objections with numbers. But surely it's up to Mark to show numbers
> first?

+1. While it would be helpful for criticisms of the PEP to be more specific and provide more of a cost-benefit analysis, the burden of proof for demonstrating the benefits provided are ultimately up to the author(s) of the PEP. We require far more justification for making a change to impose the limits in the first place than we do for maintaining the status quo.

Personally, I don't think it would be reasonable to impose any of these limits without some form of concrete evidence that doing so will provide an improvement; in any combination of efficiency, performance, and/or security, as the PEP suggests would happen. But, I don't think exact numbers are needed. (I.E. adding these limits will improve performance across all Python programs by an average of 22.359%!).

On Mon, Dec 9, 2019 at 7:15 PM Chris Angelico <rosuav@gmail.com> wrote:
On Tue, Dec 10, 2019 at 10:51 AM Steven D'Aprano <steve@pearwood.info> wrote:
>
> On Mon, Dec 09, 2019 at 02:12:37PM -0800, Nathaniel Smith wrote:
> > > > On 09/12/2019 2:15 pm, Chris Angelico wrote:
> > > You: "We should limit things. Stuff will be faster."
> > > Others: "Really? Because bit masking is work. It'll be slower."
>
> I'm not an expert, but the impression I've got from various discussions
> on performance over the last few years is that the single biggest
> bottleneck for CPU performance is memory locality. Cache misses are so
> expensive, and CPU instructions so fast, that memory locality is king
> and the cost of bit masking is insignificant. In other words, worrying
> about the cost of bit masking in C code is so very 1990s.
>
> I could be wrong of course: I'm not an expert. And I don't think we
> should take it for granted that this is the case, unless some experts on
> modern CPUs speak up and say that Mark is so obviously correct that a
> demonstration is unnecessary.

And the speculation continues.

> > > You: "Maybe we limit it somewhere else, whatever. It'll be faster."
>
> That's a totally unfair and inaccurate representation of Mark's
> position. The PEP doesn't say "let's put in arbitrary limits in random
> places for the lols", he proposed seven concrete limits and gave reasons
> for why he expects that they will improve memory efficiency, safety,
> performance or all three.

*He expects that*. That's what I'm talking about. We have lots of
speculation and no evidence either way.

> Having said that though, I think you are right that the PEP could do
> with a bit more detail on the current status quo and existing limits,
> and how the proposed changes will improve safety and memory use.

Exactly. Yes, I know that I massively oversimplified things in that
post. But you nonetheless acknowledge here that we are *still* quite
lacking in any actual evidence. We have people who believe that a bit
mask will slow things down, others who claim that improved cache
locality will speed things up, and Mark asks us to please justify our
objections with numbers. But surely it's up to Mark to show numbers
first?

ChrisA
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