Re: Removing pure space

[Pip Cet <pipcet@gmail.com>]

On Thu, Mar 4, 2021 at 2:49 PM Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> > I have not been able to measure any performance changes with this
> > patch applied, but unfortunately I'm currently on a system unsuitable
> > for running reliable benchmarks. If this is a concern, any help
> > measuring the performance impact more accurately would be appreciated.
>
> The main expected benefit from the purespace is that the GC doesn't need
> to look at it, so the GC should be faster because it looks at a smaller
> part of the heap.

With pdumper, I don't think that's true anymore. There are three Lisp
objects for which PURE_P returns true: the empty vector, unibyte
string, and multibyte string.

In fact, if anything, it'll be slower because of the extra overhead of
the PURE_P checks...

> So I think a good micro-benchmark which should expose the worst-case
> impact of removing the purespace would be to compare the time taken to
> perform GC (I think the effect should be most visible right at startup
> since the more packages and stuff you have loaded, the smaller the
> proportion of the heap kept in purespace).

So you're saying I should build with unexec, then run something like:

"time ./emacs --batch --eval '(dotimes (i 10000) (garbage-collect))'

? Because I don't really think that benchmark makes sense with pdumper...

Pip