Re: pdumper's performance

[Stefan Monnier]

Thanks Daniel for your prompt response.  I have some further questions, tho.

> You can see for yourself whether there's an impact. Compile an Emacs with
> support for both pdumper and unexec, dump it with unexec, and compare its
> GC performance to Emacs built without support for pdumper and also dumped
> with unexec.

I hoping to save myself the time ;-)
[ BTW, part of the reason for those questions is that I'm writing an
  article about the history of Elisp, and I'd like to understand how
  your code works so I can say something intelligent about it.
  Oh and there's not much time left before the deadline.
  Another part of course, is that I'd like to see this feature land
  on master.  ]

> As I recall, the difference is minimal.

Do you recall the tests you used and the ballpark of the difference?

>> Also I don't quite understand why this is needed: IIUC the markbits of
>> pdump'd objects are stored elsewhere, but I don't understand why that
>> needs to be the case.
> Because we don't store dumped objects in blocks and so the calculations of
> the normal locations of their mark bits would be wrong.

Hmm... OK that could explain it for conses and floats where we keep the
markbits separately from the objects in bitmaps alongside those blocs,
but you also have those <foo>_marked_p and set_<foo>_marked functions for
all other types of objects where the markbit is normally stored within
the object itself (i.e. it doesn't matter whether they're in blocks or
not).

Why did you choose to use a completely different layout for the objects
loaded from the dump?  I naively thought your code would take
cons_blocks, symbol_blocks, ... and write those blocks as-is so objects
keep the same layout, and things like mark_maybe_object don't need to be
changed at all.  I understand this would end up writing larger dumps
(since they would include some free objects), but I'd have expected it
would lead to simpler code and a smaller patch.

What am I missing?


        Stefan