Reduce concurrency when using glibc-based Linux

to help prevent memory fragmentation

lib/utility.js

@@ -1,6 +1,8 @@
 'use strict';
 
 const events = require('events');
+const detectLibc = require('detect-libc');
+
 const is = require('./is');
 const sharp = require('../build/Release/sharp.node');
 
@@ -84,8 +86,12 @@ cache(true);
 /**
  * Gets or, when a concurrency is provided, sets
  * the number of threads _libvips'_ should create to process each image.
- * The default value is the number of CPU cores.
- * A value of `0` will reset to this default.
+ *
+ * The default value is the number of CPU cores,
+ * except when using glibc-based Linux without jemalloc,
+ * where the default is `1` to help reduce memory fragmentation.
+ *
+ * A value of `0` will reset this to the number of CPU cores.
  *
  * The maximum number of images that can be processed in parallel
  * is limited by libuv's `UV_THREADPOOL_SIZE` environment variable.
@@ -103,6 +109,11 @@ cache(true);
 function concurrency (concurrency) {
   return sharp.concurrency(is.integer(concurrency) ? concurrency : null);
 }
+/* istanbul ignore next */
+if (detectLibc.family === detectLibc.GLIBC && !sharp._isUsingJemalloc()) {
+  // Reduce default concurrency to 1 when using glibc memory allocator
+  sharp.concurrency(1);
+}
 
 /**
  * An EventEmitter that emits a `change` event when a task is either: