Add a check for environmental variable TIPPECANOE_MAX_THREADS

README.md

@@ -147,6 +147,12 @@ resolution is obtained than by using a smaller _maxzoom_ or _detail_.
  * -pt or --no-tiny-polygon-reduction: Don't combine the area of very small polygons into small squares that represent their combined area.
  * -q or --quiet: Work quietly instead of reporting progress
 
+Environment
+-----------
+
+Tippecanoe ordinarily uses as many parallel threads as the operating system claims that CPUs are available.
+You can override this number by setting the `TIPPECANOE_MAX_THREADS` environmental variable.
+
 Example
 -------
 

main.cpp

@@ -117,7 +117,14 @@ void checkdisk(struct reader *r, int nreader) {
 };
 
 void init_cpus() {
+	const char *TIPPECANOE_MAX_THREADS = getenv("TIPPECANOE_MAX_THREADS");
+
+	if (TIPPECANOE_MAX_THREADS != NULL) {
+		CPUS = atoi(TIPPECANOE_MAX_THREADS);
+	} else {
 		CPUS = sysconf(_SC_NPROCESSORS_ONLN);
+	}
+
 	if (CPUS < 1) {
 		CPUS = 1;
 	}

man/tippecanoe.1

@@ -188,6 +188,10 @@ which may not be what you want.
 .IP \(bu 2
 \-q or \-\-quiet: Work quietly instead of reporting progress
 .RE
+.SH Environment
+.PP
+Tippecanoe ordinarily uses as many parallel threads as the operating system claims that CPUs are available.
+You can override this number by setting the \fB\fCTIPPECANOE_MAX_THREADS\fR environmental variable.
 .SH Example
 .PP
 .RS