Introduce write-mostly sums

wmsum counters are a reduced version of aggsum counters, optimized for
write-mostly scenarios.  They do not provide optimized read functions,
but instead allow much cheaper add function.  The primary usage is
infrequently read statistic counters, not requiring exact precision.

The Linux implementation is directly mapped into percpu_counter KPI.
The FreeBSD implementation is directly mapped into counter(9) KPI.
In user-space due to lack of better implementation mapped to aggsum.

Unfortunately neither Linux percpu_counter nor FreeBSD counter(9)
provide sufficient functionality to completelly replace aggsum, so
it still remains to be used for several hot counters.

Reviewed-by: Paul Dagnelie <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: Ryan Moeller <[email protected]>
Signed-off-by: Alexander Motin <[email protected]>
Sponsored-By: iXsystems, Inc.
Closes #12114

2 files changed, 69 insertions, 0 deletions

diff --git a/lib/libspl/include/sys/Makefile.am b/lib/libspl/include/sys/Makefile.am
index 53a36f6bf..6816a0125 100644
--- a/lib/libspl/include/sys/Makefile.am
+++ b/lib/libspl/include/sys/Makefile.am
@@ -44,4 +44,5 @@ libspl_HEADERS = \
 	varargs.h \
 	vnode.h \
 	vtoc.h \
+	wmsum.h \
 	zone.h
diff --git a/lib/libspl/include/sys/wmsum.h b/lib/libspl/include/sys/wmsum.h
new file mode 100644
index 000000000..0679af73c
--- /dev/null
+++ b/lib/libspl/include/sys/wmsum.h
@@ -0,0 +1,68 @@
+/*
+ * CDDL HEADER START
+ *
+ * This file and its contents are supplied under the terms of the
+ * Common Development and Distribution License ("CDDL"), version 1.0.
+ * You may only use this file in accordance with the terms of version
+ * 1.0 of the CDDL.
+ *
+ * A full copy of the text of the CDDL should have accompanied this
+ * source.  A copy of the CDDL is also available via the Internet at
+ * http://www.illumos.org/license/CDDL.
+ *
+ * CDDL HEADER END
+ */
+
+/*
+ * wmsum counters are a reduced version of aggsum counters, optimized for
+ * write-mostly scenarios.  They do not provide optimized read functions,
+ * but instead allow much cheaper add function.  The primary usage is
+ * infrequently read statistic counters, not requiring exact precision.
+ *
+ * In user-space due to lack of better implementation mapped to aggsum.
+ */
+
+#ifndef	_SYS_WMSUM_H
+#define	_SYS_WMSUM_H
+
+#include <sys/aggsum.h>
+
+#ifdef	__cplusplus
+extern "C" {
+#endif
+
+#define	wmsum_t	aggsum_t
+
+static inline void
+wmsum_init(wmsum_t *ws, uint64_t value)
+{
+
+	aggsum_init(ws, value);
+}
+
+static inline void
+wmsum_fini(wmsum_t *ws)
+{
+
+	aggsum_fini(ws);
+}
+
+static inline uint64_t
+wmsum_value(wmsum_t *ws)
+{
+
+	return (aggsum_value(ws));
+}
+
+static inline void
+wmsum_add(wmsum_t *ws, int64_t delta)
+{
+
+	aggsum_add(ws, delta);
+}
+
+#ifdef	__cplusplus
+}
+#endif
+
+#endif /* _SYS_WMSUM_H */