Add a parameter to limit how big chunks we attempt to allocate

from storage.  Asking for more than 256M at a time will not
actually improve performance and is quite likely to hurt it.

bin/varnishd/heritage.h

@@ -118,6 +118,7 @@ struct params {
 
 	/* Fetcher hints */
 	unsigned		fetch_chunksize;
+	unsigned		fetch_maxchunksize;
 
 #ifdef SENDFILE_WORKS
 	/* Sendfile object minimum size */

bin/varnishd/mgt_param.c

@@ -629,6 +629,13 @@ static const struct parspec input_parspec[] = {
 		"above 128kb a dubious idea.",
 		EXPERIMENTAL,
 		"128", "kilobytes" },
+	{ "fetch_maxchunksize",
+		tweak_uint, &master.fetch_maxchunksize, 64, UINT_MAX / 1024.,
+		"The maximum chunksize we attempt to allocate from storage. "
+		"Making this too large may cause delays and storage "
+		"fragmentation.\n",
+		EXPERIMENTAL,
+		"262144", "kilobytes" },
 #ifdef SENDFILE_WORKS
 	{ "sendfile_threshold",
 		tweak_uint, &master.sendfile_threshold, 0, UINT_MAX,

bin/varnishd/stevedore.c

@@ -171,6 +171,9 @@ stv_alloc(const struct sess *sp, size_t size)
 	}
 	CHECK_OBJ_NOTNULL(stv, STEVEDORE_MAGIC);
 
+	if (size > (size_t)(params->fetch_maxchunksize) << 10)
+		size = (size_t)(params->fetch_maxchunksize) << 10;
+
 	for (;;) {
 		/* try to allocate from it */
 		AN(stv->alloc);