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Advise against using too-high GC thresholds

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* doc/lispref/internals.texi (Garbage Collection):
* src/alloc.c (syms_of_alloc) <gc-cons-threshold>
<gc-cons-percentage>: Advise against enlarging the GC thresholds
more than needed and for prolonged periods of time.
This commit is contained in:
Eli Zaretskii 2022-11-20 13:10:08 +02:00
parent fbad9c05ea
commit 7b09ed28ba
2 changed files with 25 additions and 7 deletions
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20
doc/lispref/internals.texi
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@ -554,12 +554,17 @@ trigger another garbage collection. You can use the result returned by
object type; space allocated to the contents of buffers does not count.
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
is 400,000 for the default 32-bit configuration and 800,000 for the 64-bit
one. If you specify a larger value, garbage collection will happen less
often. This reduces the amount of time spent garbage collecting, but
increases total memory use. You may want to do this when running a program
that creates lots of Lisp data.
@file{alloc.c}. Since it's defined in @code{word_size} units, the
value is 400,000 for the default 32-bit configuration and 800,000 for
the 64-bit one. If you specify a larger value, garbage collection
will happen less often. This reduces the amount of time spent garbage
collecting, but increases total memory use. You may want to do this
when running a program that creates lots of Lisp data. However, we
recommend against increasing the threshold for prolonged periods of
time, and advise that you never set it higher than needed for the
program to run in reasonable time. Using thresholds higher than
necessary could potentially cause system-wide memory pressure, and
should therefore be avoided.
You can make collections more frequent by specifying a smaller value, down
to 1/10th of @code{GC_DEFAULT_THRESHOLD}. A value less than this minimum
@ -576,6 +581,9 @@ garbage collection occurs only when both criteria are satisfied.
As the heap size increases, the time to perform a garbage collection
increases. Thus, it can be desirable to do them less frequently in
proportion.
As with @code{gc-cons-threshold}, do not enlarge this more than
necessary, and never for prolonged periods of time.
@end defopt
Control over the garbage collector via @code{gc-cons-threshold} and

12
src/alloc.c
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@ -7775,13 +7775,23 @@ allocated since the last garbage collection. All data types count.
Garbage collection happens automatically only when `eval' is called.
By binding this temporarily to a large number, you can effectively
prevent garbage collection during a part of the program.
prevent garbage collection during a part of the program. But be
sure to get back to the normal value soon enough, to avoid system-wide
memory pressure, and never use a too-high value for prolonged periods
of time.
See also `gc-cons-percentage'. */);
DEFVAR_LISP ("gc-cons-percentage", Vgc_cons_percentage,
doc: /* Portion of the heap used for allocation.
Garbage collection can happen automatically once this portion of the heap
has been allocated since the last garbage collection.
By binding this temporarily to a large number, you can effectively
prevent garbage collection during a part of the program. But be
sure to get back to the normal value soon enough, to avoid system-wide
memory pressure, and never use a too-high value for prolonged periods
of time.
If this portion is smaller than `gc-cons-threshold', this is ignored. */);
Vgc_cons_percentage = make_float (0.1);