bug#37006: 27.0.50; garbage collection not happening after 26de2d42

[Paul Eggert <eggert@cs.ucla.edu>]

Eli Zaretskii wrote:

> However, I'd rather we don't invent new data types unless really
> necessary.

I did that yesterday, in commit 2019-08-13T19:20:40Z!eggert@cs.ucla.edu 
(b80559be212292d44ce14ca5e94505cab4d9a868).

> gc-cons-threshold is a Lisp integer, a
> fixnum, so it cannot exceed EMACS_INT_MAX, I think.

No, (setq gc-cons-threshold (1+ most-positive-fixnum)) works and does the right 
thing. The variable's value can be any intmax_t value. This is useful for 
quantities like GC object byte counts that might not fit into fixnums.

> Can we use for this purpose the existing trapped_write
> field of Lisp_Symbol that is the base for implementing Lisp watcher
> functions?

Don't see why not.

> With the old code, whenever memory-full was non-nil, and
> consing_since_gc was more than the size of cons_block (about 1KB on my
> system), the very next maybe_gc call would actually trigger GC.  With
> the new code, no matter how much consing happened before memory-full
> became non-nil, we still need to cons 1KB worth of objects before GC
> happens.  This 1KB might be critical when we are out of memory.

I don't think the scenario is worth worrying about doing a GC now rather than 
later. But if we go the trapped_write route, this issue won't matter since the 
GC will be done quickly.

>> Immediate-GC might cause GC thrashing, no?
> 
> Not sure how, can you elaborate?

When EMacs is low on memory, if we're not careful Emacs could GC every time 
maybe_gc is called, which will be roughly equivalent to Emacs hanging and doing 
nothing.