[lttng-dev] [PATCH 1/6] change default mm type

[Lai Jiangshan]

When I test backend with the following commands.
(my box is x86_64 with 4 cores/logic cpus)
**(test with Load factor = 100% only)**

./tests/test_urcu_hash 4 0 10 -B mmap -h $((1<<19)) -p $((1<<19))
./tests/test_urcu_hash 4 0 10 -B mmap -h $((1<<18)) -p $((1<<18))
./tests/test_urcu_hash 4 0 10 -B mmap -h $((1<<17)) -p $((1<<17))
./tests/test_urcu_hash 4 0 10 -B mmap -h $((1<<16)) -p $((1<<16))
                    4readers/no writer

It shows that mmap backend is about 6% better over order backend.
(It also shows that chunk backend is (worse than)/(the same as) order backend
for small/large min_nr_alloc_buckets. (use -m when test)).

Note:
"6%" and the google-perftools told us the bucket_at() is not the cirtical
bottle neck.

new strategy:
infinite buckets size --> order mm
otherwise 64bits      --> mmap mm
otherwise    --> order or chunk mm

Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
---
 rculfhash.c      |   14 ++++++++++++++
 urcu/rculfhash.h |    2 +-
 2 files changed, 15 insertions(+), 1 deletions(-)

diff --git a/rculfhash.c b/rculfhash.c
index 8c835de..ebf0023 100644
--- a/rculfhash.c
+++ b/rculfhash.c
@@ -1280,6 +1280,9 @@ struct cds_lfht *_cds_lfht_new(unsigned long init_size,
 	if (!init_size || (init_size & (init_size - 1)))
 		return NULL;
 
+	if (!max_nr_buckets && !mm)
+		mm = &cds_lfht_mm_order;
+
 	/* max_nr_buckets == 0 for order based mm means infinite */
 	if (mm == &cds_lfht_mm_order && !max_nr_buckets)
 		max_nr_buckets = 1UL << (MAX_TABLE_ORDER - 1);
@@ -1293,6 +1296,17 @@ struct cds_lfht *_cds_lfht_new(unsigned long init_size,
 	max_nr_buckets = max(max_nr_buckets, min_nr_alloc_buckets);
 	init_size = min(init_size, max_nr_buckets);
 
+	if (!mm) {
+		if (CAA_BITS_PER_LONG > 32) {
+			mm = &cds_lfht_mm_mmap;
+		} else if (max_nr_buckets / min_nr_alloc_buckets
+				<= MAX_TABLE_ORDER) {
+			mm = &cds_lfht_mm_chunk;
+		} else {
+			mm = &cds_lfht_mm_order;
+		}
+	}
+
 	ht = mm->alloc_cds_lfht(min_nr_alloc_buckets, max_nr_buckets);
 	assert(ht);
 	assert(ht->mm == mm);
diff --git a/urcu/rculfhash.h b/urcu/rculfhash.h
index 6ed7c8c..de31526 100644
--- a/urcu/rculfhash.h
+++ b/urcu/rculfhash.h
@@ -154,7 +154,7 @@ struct cds_lfht *cds_lfht_new(unsigned long init_size,
 			pthread_attr_t *attr)
 {
 	return _cds_lfht_new(init_size, min_nr_alloc_buckets, max_nr_buckets,
-			flags, &cds_lfht_mm_order, &rcu_flavor, attr);
+			flags, NULL, &rcu_flavor, attr);
 }
 
 /*
-- 
1.7.4.4