Add per-thread sharded histograms for heavy loads

Although an `isc_histo_t` is thread-safe, it can suffer
from cache contention under heavy load. To avoid this,
an `isc_histomulti_t` contains a histogram per thread,
so updates are local and low-contention.

lib/isc/histo.c

@@ -25,6 +25,7 @@
 #include <isc/histo.h>
 #include <isc/magic.h>
 #include <isc/mem.h>
+#include <isc/tid.h>
 
 /*
  * XXXFANF to be added to isc/util.h by a commmit in a qp-trie
@@ -33,8 +34,10 @@
 #define STRUCT_FLEX_SIZE(pointer, member, count) \
 	(sizeof(*(pointer)) + sizeof(*(pointer)->member) * (count))
 
-#define HISTO_MAGIC    ISC_MAGIC('H', 's', 't', 'o')
-#define HISTO_VALID(p) ISC_MAGIC_VALID(p, HISTO_MAGIC)
+#define HISTO_MAGIC	    ISC_MAGIC('H', 's', 't', 'o')
+#define HISTO_VALID(p)	    ISC_MAGIC_VALID(p, HISTO_MAGIC)
+#define HISTOMULTI_MAGIC    ISC_MAGIC('H', 'g', 'M', 't')
+#define HISTOMULTI_VALID(p) ISC_MAGIC_VALID(p, HISTOMULTI_MAGIC)
 
 /*
  * Natural logarithms of 2 and 10 for converting precisions between
@@ -101,6 +104,12 @@ struct isc_histosummary {
 	uint64_t buckets[];
 };
 
+struct isc_histomulti {
+	uint magic;
+	uint size;
+	isc_histo_t *hg[];
+};
+
 /**********************************************************************/
 
 #define OUTARG(ptr, val)                \
@@ -402,6 +411,67 @@ isc_histo_merge(isc_histo_t **targetp, isc_historead_t source) {
 
 /**********************************************************************/
 
+void
+isc_histomulti_create(isc_mem_t *mctx, uint sigbits, isc_histomulti_t **hmp) {
+	REQUIRE(hmp != NULL);
+	REQUIRE(*hmp == NULL);
+
+	uint size = isc_tid_count();
+	INSIST(size > 0);
+
+	isc_histomulti_t *hm = isc_mem_getx(
+		mctx, STRUCT_FLEX_SIZE(hm, hg, size), ISC_MEM_ZERO);
+	*hm = (isc_histomulti_t){
+		.magic = HISTOMULTI_MAGIC,
+		.size = size,
+	};
+
+	for (uint i = 0; i < hm->size; i++) {
+		isc_histo_create(mctx, sigbits, &hm->hg[i]);
+	}
+
+	*hmp = hm;
+}
+
+void
+isc_histomulti_destroy(isc_histomulti_t **hmp) {
+	REQUIRE(hmp != NULL);
+	REQUIRE(HISTOMULTI_VALID(*hmp));
+
+	isc_histomulti_t *hm = *hmp;
+	isc_mem_t *mctx = hm->hg[0]->mctx;
+	*hmp = NULL;
+
+	for (uint i = 0; i < hm->size; i++) {
+		isc_histo_destroy(&hm->hg[i]);
+	}
+
+	isc_mem_put(mctx, hm, STRUCT_FLEX_SIZE(hm, hg, hm->size));
+}
+
+void
+isc_histomulti_merge(isc_histo_t **hgp, isc_histomulti_t *hm) {
+	REQUIRE(HISTOMULTI_VALID(hm));
+
+	for (uint i = 0; i < hm->size; i++) {
+		isc_histo_merge(hgp, hm->hg[i]);
+	}
+}
+
+void
+isc_histomulti_add(isc_histomulti_t *hm, uint64_t value, uint64_t inc) {
+	REQUIRE(HISTOMULTI_VALID(hm));
+	isc_histo_t *hg = hm->hg[isc_tid()];
+	add_key_count(hg, value_to_key(hg, value), inc);
+}
+
+void
+isc_histomulti_inc(isc_histomulti_t *hm, uint64_t value) {
+	isc_histomulti_add(hm, value, 1);
+}
+
+/**********************************************************************/
+
 /*
  * https://fanf2.user.srcf.net/hermes/doc/antiforgery/stats.pdf
  * equation 4 (incremental mean) and equation 44 (incremental variance)

lib/isc/include/isc/histo.h

@@ -72,6 +72,7 @@
 
 typedef struct isc_histo	isc_histo_t;
 typedef struct isc_histosummary isc_histosummary_t;
+typedef struct isc_histomulti	isc_histomulti_t;
 
 /*
  * For functions that can take either type.
@@ -157,6 +158,8 @@ isc_histo_digits_to_bits(uint digits);
  *\li	`digits <= ISC_HISTO_MAXDIGS`
  */
 
+/**********************************************************************/
+
 void
 isc_histo_inc(isc_histo_t *hg, uint64_t value);
 /*%<
@@ -269,6 +272,76 @@ isc_histo_merge(isc_histo_t **targetp, isc_historead_t source);
 
 /**********************************************************************/
 
+void
+isc_histomulti_create(isc_mem_t *mctx, uint sigbits, isc_histomulti_t **hmp);
+/*%<
+ * Create a multithreaded sharded histogram.
+ *
+ * Although an `isc_histo_t` is thread-safe, it can suffer
+ * from cache contention under heavy load. To avoid this,
+ * an `isc_histomulti_t` contains a histogram per thread,
+ * so updates are local and low-contention.
+ *
+ * Requires:
+ *\li	`sigbits >= ISC_HISTO_MINBITS`
+ *\li	`sigbits <= ISC_HISTO_MAXBITS`
+ *\li	`hmp != NULL`
+ *\li	`*hmp == NULL`
+ *
+ * Ensures:
+ *\li	`*hmp` is a pointer to a multithreaded sharded histogram.
+ */
+
+void
+isc_histomulti_destroy(isc_histomulti_t **hmp);
+/*%<
+ * Destroy a multithreaded sharded histogram
+ *
+ * Requires:
+ *\li	`hmp != NULL`
+ *\li	`*hmp` is a pointer to a valid multithreaded sharded histogram
+ *
+ * Ensures:
+ *\li	all memory allocated by the histogram has been released
+ *\li	`*hmp == NULL`
+ */
+
+void
+isc_histomulti_merge(isc_histo_t **targetp, isc_histomulti_t *source);
+/*%<
+ * Increase the counts in `*targetp` by the counts recorded in `source`
+ *
+ * The target histogram is created if `*targetp` is NULL.
+ *
+ * Requires:
+ *\li	`targetp != NULL`
+ *\li	`*targetp` is NULL or a pointer to a valid histogram
+ *\li	`source` is a pointer to a valid multithreaded sharded histogram
+ *
+ * Ensures:
+ *\li	`*targetp` is a pointer to a valid histogram
+ */
+
+void
+isc_histomulti_inc(isc_histomulti_t *hm, uint64_t value);
+/*%<
+ * Add 1 to the value's bucket
+ *
+ * Requires:
+ *\li	`hm` is a pointer to a valid histomulti
+ */
+
+void
+isc_histomulti_add(isc_histomulti_t *hm, uint64_t value, uint64_t inc);
+/*%<
+ * Add an arbitrary increment to the value's bucket
+ *
+ * Requires:
+ *\li	`hm` is a pointer to a valid histomulti
+ */
+
+/**********************************************************************/
+
 void
 isc_histosummary_create(const isc_histo_t *hg, isc_histosummary_t **hsp);
 /*%<
@@ -406,3 +479,5 @@ isc_histo_cdf(const isc_histosummary_t *hs, uint64_t value,
  *\li	`hs` is a pointer to a valid histogram summary
  *\li	`proportionp != NULL`
  */
+
+/**********************************************************************/

lib/isc/include/isc/tid.h

@@ -21,6 +21,12 @@ ISC_LANG_BEGINDECLS
 
 #define ISC_TID_UNKNOWN UINT32_MAX
 
+uint32_t
+isc_tid_count(void);
+/*%<
+ * Returns the number of threads.
+ */
+
 uint32_t
 isc_tid(void);
 /*%<
@@ -32,4 +38,7 @@ isc_tid(void);
 void
 isc__tid_init(uint32_t tid);
 
+void
+isc__tid_initcount(uint32_t count);
+
 ISC_LANG_ENDDECLS

lib/isc/loop.c

@@ -346,6 +346,7 @@ isc_loopmgr_create(isc_mem_t *mctx, uint32_t nloops, isc_loopmgr_t **loopmgrp) {
 	REQUIRE(nloops > 0);
 
 	threadpool_initialize(nloops);
+	isc__tid_initcount(nloops);
 
 	loopmgr = isc_mem_get(mctx, sizeof(*loopmgr));
 	*loopmgr = (isc_loopmgr_t){

lib/isc/tid.c

@@ -26,7 +26,13 @@
 
 #define ISC_TID_UNKNOWN UINT32_MAX
 
-static thread_local uint32_t isc__tid_v = ISC_TID_UNKNOWN;
+static thread_local uint32_t tid_local = ISC_TID_UNKNOWN;
+
+/*
+ * Zero is a better nonsense value in this case than ISC_TID_UNKNOWN;
+ * avoids things like trying to allocate 32GB of per-thread counters.
+ */
+static uint32_t tid_count = 0;
 
 /**
  * Protected
@@ -34,9 +40,14 @@ static thread_local uint32_t isc__tid_v = ISC_TID_UNKNOWN;
 
 void
 isc__tid_init(uint32_t tid) {
-	REQUIRE(isc__tid_v == ISC_TID_UNKNOWN || isc__tid_v == tid);
+	REQUIRE(tid_local == ISC_TID_UNKNOWN || tid_local == tid);
+	tid_local = tid;
+}
 
-	isc__tid_v = tid;
+void
+isc__tid_initcount(uint32_t count) {
+	REQUIRE(tid_count == 0 || tid_count == count);
+	tid_count = count;
 }
 
 /**
@@ -45,5 +56,10 @@ isc__tid_init(uint32_t tid) {
 
 uint32_t
 isc_tid(void) {
-	return (isc__tid_v);
+	return (tid_local);
+}
+
+uint32_t
+isc_tid_count(void) {
+	return (tid_count);
 }