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Improve doc of GC thresholds

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* doc/lispref/internals.texi (Garbage Collection), etc/NEWS:
Warn that control over GC is only approximate.
This commit is contained in:
Paul Eggert 2019-09-14 10:53:24 -07:00
parent 3f43adac49
commit 1acc0cc9aa
2 changed files with 17 additions and 3 deletions
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13
doc/lispref/internals.texi
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@ -533,9 +533,6 @@ be allocated for Lisp objects after one garbage collection in order to
trigger another garbage collection. You can use the result returned by
@code{garbage-collect} to get an information about size of the particular
object type; space allocated to the contents of buffers does not count.
Note that the subsequent garbage collection does not happen immediately
when the threshold is exhausted, but only the next time the Lisp interpreter
is called.
The initial threshold value is @code{GC_DEFAULT_THRESHOLD}, defined in
@file{alloc.c}. Since it's defined in @code{word_size} units, the value
@ -562,6 +559,16 @@ increases. Thus, it can be desirable to do them less frequently in
proportion.
@end defopt
Control over the garbage collector via @code{gc-cons-threshold} and
@code{gc-cons-percentage} is only approximate. Although Emacs checks
for threshold exhaustion regularly, for efficiency reasons it does not
do so immediately after every change to the heap or to
@code{gc-cons-threshold} or @code{gc-cons-percentage}, so exhausting
the threshold does not immediately trigger garbage collection. Also,
for efficency in threshold calculations Emacs approximates the heap
size, which counts the bytes used by currently-accessible objects in
the heap.
The value returned by @code{garbage-collect} describes the amount of
memory used by Lisp data, broken down by data type. By contrast, the
function @code{memory-limit} provides information on the total amount of

7
etc/NEWS
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@ -2429,6 +2429,13 @@ remote systems, which support this check.
+++
** 'memory-limit' now returns a better estimate of memory consumption.
+++
** When interpreting 'gc-cons-percentage', Emacs now estimates the
heap size more often and (we hope) more accurately. E.g., formerly
(progn (let ((gc-cons-percentage 0.8)) BODY1) BODY2) continued to use
the 0.8 value during BODY2 until the next garbage collection, but that
is no longer true. Applications may need to re-tune their GC tricks.
+++
** New macro 'combine-change-calls' arranges to call the change hooks
('before-change-functions' and 'after-change-functions') just once