Re: MPS: dangling markers

["Gerd Möllmann" <gerd.moellmann@gmail.com>]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

>> Can it be that `remove_marker` is called much more often than add_marker?
>
> I don't think you can remove a marker before having added it, so it
> seems rather unlikely.
>
>>   igc_add_marker (struct buffer *b, struct Lisp_Marker *m)
>>   {
>>     Lisp_Object v = BUF_MARKERS (b);
>>     if (NILP (v))
>>       v = BUF_MARKERS (b) = alloc_vector_weak (1, Qnil);
>>
>>     ptrdiff_t i = find_nil_index (v);
>
> My guess is that `find_nil_index` almost always scans the buffer until
> near the end.  It should be fairly easy to speed that up by keeping
> a "pointer" to the last known empty slot, or use the empty slots to
> "point to each other" to form a free-list.

That could be an idea, indeed.

>
> Still doesn't explain why `remove_marker` takes more time on your branch
> than on `master`, unless we end up keeping significantly larger vectors
> than the length of the linked-lists used on `master`.
>
> Maybe it's because `unchain_marker` often exits early (e.g. maybe it's
> common that `unchain_marker` is used to delete a marker recently added,
> so it's near the top of the linked list which ends up behaving a bit
> like a stack)?

And that makes sense. I'll try tomorrow to add something to igc that
Ihor can tell us how many markers there are that are allocated in the
place Eli showed.