Fix and properly document reserve limits

Could have caused #5, related to #10

src/fellow_tune.c

@@ -63,21 +63,6 @@ stvfe_tune_check(struct stvfe_tune *tune)
 
 	assert (tune->dsksz >= tune->memsz);
 
-#ifdef DOES_THIS_MAKE_SENSE
-	/* the average object could be quite small, but larger chunks
-	 * could be required, in which case it makes sense for the LRU
-	 * to keep the larger chunks in the reserve
-	 */
-	l = log2up(tune->objsize_hint);
-	if (tune->chunk_exponent > l) {
-		fprintf(stderr, "fellow: chunk_exponent limited to %u "
-		    "(next pow2 of objsize_hint)\n", l);
-		tune->mem_reserve_chunks <<= (tune->chunk_exponent - l);
-		tune->dsk_reserve_chunks <<= (tune->chunk_exponent - l);
-		tune->chunk_exponent = l;
-	}
-#endif
-
 #define CHUNK_SHIFT 10
 
 	l = log2down(tune->memsz);
@@ -97,7 +82,7 @@ stvfe_tune_check(struct stvfe_tune *tune)
 		tune->chunk_exponent = l;
 	}
 
-	sz = tune->memsz >> (tune->chunk_exponent - 1);
+	sz = tune->memsz >> (tune->chunk_exponent + 3);
 	assert(sz <= UINT_MAX);
 	l = (unsigned)sz;
 	if (tune->mem_reserve_chunks > l) {
@@ -106,6 +91,15 @@ stvfe_tune_check(struct stvfe_tune *tune)
 		tune->mem_reserve_chunks = l;
 	}
 
+	sz = tune->dsksz >> (tune->chunk_exponent + 3);
+	assert(sz <= UINT_MAX);
+	l = (unsigned)sz;
+	if (tune->dsk_reserve_chunks > l) {
+		fprintf(stderr,"fellow: dsk_reserve_chunks limited to %u "
+		    "(less than 1/8 of disk size)\n", l);
+		tune->dsk_reserve_chunks = l;
+	}
+
 //lint --e{685,568} misc const comparisons
 #define TUNE(t, n, d, min, max)						\
 	if (tune->n < (min))						\

src/vmod_slash.vcc

@@ -529,6 +529,7 @@ fellow storage can be fine tuned:
   - unit: scalar
   - default: 4
   - minimum: 2
+  - maximum: dsksize / 8 / chunk_bytes
 
   specifies a number of chunks to reserve on disk. The reserve is used
   to fulfill storage requests when storage is otherwise full. Because
@@ -538,11 +539,15 @@ fellow storage can be fine tuned:
   fixed log space is full, might momentarily require additional space
   before making room.
 
+  The value is capped suck that the number of reserved chunks times
+  the chunk size does not exceed 1/8 of the disk size.
+
 * *mem_reserve_chunks*
 
   - unit: scalar
   - default: 1
   - minimum: 0
+  - maximum: memsize / 8 / chunk_bytes
 
   specifies a number of chunks to reserve in memory. The reserve is
   used to provide memory for new objects or objects staged from disk
@@ -550,6 +555,9 @@ fellow storage can be fine tuned:
   latencies in these situations at the expense of some memory
   unavailable for caching.
 
+  The value is capped suck that the number of reserved chunks times
+  the chunk size does not exceed 1/8 of the memory size.
+
 * *objsize_hint*
 
   - unit: bytes

src/vtc/fellow_vcl.vtc

@@ -12,7 +12,7 @@ varnish v1 -arg "-p debug=+vclrel -p vsl_mask=+expkill" -vcl {
 			dsksize	=  100MB,
 			memsize	=   1MB,
 			objsize_hint	=  64KB);
-		fellow.tune(chunk_bytes = 256KB);
+		fellow.tune(chunk_bytes = 256KB, mem_reserve_chunks = 999999, dsk_reserve_chunks = 99999);
 		fellow.as_transient();
 	}
 	sub vcl_synth {