Let default arc_c_max be platform dependent

Linux changed the default max ARC size to 1/2 of physical memory to
deal with shortcomings of the Linux SLUB allocator.  Other platforms
do not require the same logic.

Implement an arc_default_max() function to determine a default max ARC
size in platform code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes openzfs#10155

include/sys/arc.h

@@ -298,6 +298,7 @@ void arc_tempreserve_clear(uint64_t reserve);
 int arc_tempreserve_space(spa_t *spa, uint64_t reserve, uint64_t txg);
 
 uint64_t arc_all_memory(void);
+uint64_t arc_default_max(uint64_t min, uint64_t allmem);
 uint64_t arc_target_bytes(void);
 void arc_init(void);
 void arc_fini(void);

man/man5/zfs-module-parameters.5

@@ -854,8 +854,11 @@ Default value: \fB10\fR%.
 \fBzfs_arc_max\fR (ulong)
 .ad
 .RS 12n
-Max arc size of ARC in bytes. If set to 0 then it will consume 1/2 of system
-RAM. This value must be at least 67108864 (64 megabytes).
+Max size of ARC in bytes.  If set to 0 then the max size of ARC is determined
+by the amount of system memory installed.  For Linux, 1/2 of system memory will
+be used as the limit.  For FreeBSD, the larger of all system memory - 1GB or
+5/8 of system memory will be used as the limit.  This value must be at least
+67108864 (64 megabytes).
 .sp
 This value can be changed dynamically with some caveats. It cannot be set back
 to 0 while running and reducing it below the current ARC size will not cause

module/os/linux/zfs/arc_os.c

@@ -60,6 +60,23 @@
 int64_t last_free_memory;
 free_memory_reason_t last_free_reason;
 
+/*
+ * Return a default max arc size based on the amount of physical memory.
+ */
+uint64_t
+arc_default_max(uint64_t min, uint64_t allmem)
+{
+	uint64_t max;
+
+	/* set max to 3/4 of all memory, or all but 1GB, whichever is more */
+	if (allmem >= 1 << 30)
+		max = allmem - (1 << 30);
+	else
+		max = min;
+
+	return (MAX(allmem * 3 / 4, max));
+}
+
 #ifdef _KERNEL
 /*
  * Return maximum amount of memory that we could possibly use.  Reduced

module/zfs/arc.c

@@ -7150,14 +7150,11 @@ arc_init(void)
 	arc_lowmem_init();
 #endif
 
-	/* Set min cache to 1/32 of all memory, or 32MB, whichever is more */
+	/* Set min cache to 1/32 of all memory, or 32MB, whichever is more. */
 	arc_c_min = MAX(allmem / 32, 2ULL << SPA_MAXBLOCKSHIFT);
-	/* set max to 3/4 of all memory, or all but 1GB, whichever is more */
-	if (allmem >= 1 << 30)
-		arc_c_max = allmem - (1 << 30);
-	else
-		arc_c_max = arc_c_min;
-	arc_c_max = MAX(allmem * 3 / 4, arc_c_max);
+
+	/* How to set default max varies by platform. */
+	arc_c_max = arc_default_max(arc_c_min, allmem);
 
 	/*
 	 * In userland, there's only the memory pressure that we artificially