adjust PLT_INCREMENTAL_GC so it can disable generational GC

A value that starts "1", "y", or "Y" enabled incremental mode
permanently (any value was allowed formerly), while a value that
starts "0", "n", or "N" causes incremental-mode requests to be
ignored.

pkgs/racket-doc/scribblings/guide/performance.scrbl

@@ -598,8 +598,9 @@ pause is as short as a minor collection's pause. Incremental mode
 tends to run more slowly overall, but it can
 provide much more consistent real-time behavior.
 
-If the @envvar{PLT_INCREMENTAL_GC} environment variable is set when
+If the @envvar{PLT_INCREMENTAL_GC} environment variable is set
-Racket starts, incremental mode is permanently enabled. Since
+to a value that starts with @litchar{1}, @litchar{y}, or @litchar{Y}
+when Racket starts, incremental mode is permanently enabled. Since
 incremental mode is only useful for certain parts of some programs,
 however, and since the need for incremental mode is a property of a
 program rather than its environment, the preferred way to enable

pkgs/racket-doc/scribblings/reference/memory.scrbl

@@ -200,7 +200,8 @@ execution. Otherwise, @racket[#f] is returned.}
 
 Set the @as-index{@envvar{PLTDISABLEGC}} environment variable (to any
 value) before Racket starts to disable @tech{garbage collection}. Set
-the @as-index{@envvar{PLT_INCREMENTAL_GC}} environment variable to
+the @as-index{@envvar{PLT_INCREMENTAL_GC}} environment variable
+to a value that starts with @litchar{1}, @litchar{y}, or @litchar{Y} to
 request incremental mode at all times, but calling
 @racket[(collect-garbage 'incremental)] in a program with a periodic
 task is generally a better mechanism for requesting incremental mode.
@@ -302,6 +303,7 @@ collection mode, the text has the format
 
 @history[#:changed "6.3.0.7" @elem{Added @envvar{PLT_INCREMENTAL_GC}.}]
 
+
 @defproc[(collect-garbage [request (or/c 'major 'minor 'incremental) 'major]) void?]{
 
 Requests an immediate @tech{garbage collection} or requests a
@@ -336,7 +338,11 @@ garbage-collection mode, depending on @racket[request]:
 
        The intent of incremental mode is to significantly reduce pause
        times due to major collections, but incremental mode typically
-       implies longer minor-collection times and higher memory use.}
+       implies longer minor-collection times and higher memory use.
+
+       If the @envvar{PLT_INCREMENTAL_GC} environment variable's value
+       starts with @litchar{0}, @litchar{n}, or @litchar{N} on
+       start-up, then incremental-mode requests are ignored.}
 
 ]
 

racket/src/racket/cmdline.inc

@@ -1542,8 +1542,23 @@ static int run_from_cmd_line(int argc, char *_argv[],
   if (getenv("PLTDISABLEGC")) {
     scheme_enable_garbage_collection(0);
   }
-  if (getenv("PLT_INCREMENTAL_GC")) {
+  {
+    char *s;
+    s = getenv("PLT_INCREMENTAL_GC");
+    if (s) {
+      if ((s[0] == '0') || (s[0] == 'n') || (s[0] == 'N'))
+        scheme_incremental_garbage_collection(0);
+      else if ((s[0] == '1') || (s[0] == 'y') || (s[0] == 'Y'))
         scheme_incremental_garbage_collection(1);
+      else {
+         PRINTF("%s: unrecognized value for PLT_INCREMENTAL_GC;\n"
+                " a value that starts \"1\", \"y\", or \"Y\" permanently enables incremental mode,\n"
+                " and a value that starts \"0\", \"n\", or \"N\" disables incremental mode,\n"
+                " and the default enables incremental mode as requested via `collect-garbage'\n"
+                "  unrecognized value: %s\n",
+                prog, s);
+      }
+    }
   }
 #endif
 

racket/src/racket/gc2/gc2.h

@@ -176,7 +176,8 @@ GC2_EXTERN void GC_request_incremental_mode(void);
   
 GC2_EXTERN void GC_set_incremental_mode(int on);
 /*
-   Sets whether incremental mode is the default. */
+   Sets whether incremental mode is the default (1), always disabled (0),
+   or available on demand (-1). */
   
 GC2_EXTERN void GC_free_all(void);
 /*

racket/src/racket/gc2/newgc.c

@@ -257,6 +257,7 @@ MAYBE_UNUSED static void GCVERBOSEprintf(NewGC *gc, const char *fmt, ...) {
 
 /* Incremental mode */
 static int always_collect_incremental_on_minor = 0;
+static int never_collect_incremental_on_minor = 0;
 #define INCREMENTAL_COLLECT_FUEL_PER_100M (4 * 1024)
 #define INCREMENTAL_REPAIR_FUEL_PER_100M  32
 
@@ -3665,17 +3666,20 @@ void GC_gcollect_minor(void)
 
 void GC_request_incremental_mode(void)
 {
+  if (!never_collect_incremental_on_minor) {
     NewGC *gc = GC_get_GC();
 
     /* The request will expire gradually, so that an extra major GC will
        be triggered if incremental mode hasn't been requested recently
        enough: */
     gc->incremental_requested = 8;
+  }
 }
 
 void GC_set_incremental_mode(int on)
 {
-  always_collect_incremental_on_minor = 1;
+  always_collect_incremental_on_minor = (on > 0);
+  never_collect_incremental_on_minor = !on;
 }
 
 void GC_enable_collection(int on)