fellow_cache: bring back smaller segment lists when not growing

src/fellow_cache.c

@@ -3002,6 +3002,7 @@ fellow_busy_body_seglist_alloc(struct fellow_busy *fbo,
 	struct fellow_cache *fc;
 	unsigned chunk_exponent;
 	uint16_t ldsegs;
+	size_t sz;
 
 	CHECK_OBJ_NOTNULL(fbo, FELLOW_BUSY_MAGIC);
 
@@ -3013,6 +3014,13 @@ fellow_busy_body_seglist_alloc(struct fellow_busy *fbo,
 	if (chunk_exponent == 0)
 		chunk_exponent = fc->tune->chunk_exponent;
 	ldsegs = fellow_cache_seglist_capacity(chunk_exponent);
+	if (fbo->growing == 0) {
+		sz = fbo->sz_estimate - fbo->sz_returned;
+		sz += (((size_t)1 << chunk_exponent) - 1);
+		sz >>= chunk_exponent;
+		if (sz < ldsegs)
+			ldsegs = (uint16_t)sz;
+	}
 
 #ifdef TEST_DRIVER
 	if (TEST_LIMIT_LDSEGS > 0)