From ea478570908b57f9c141c0082cb6b6815938ea17 Mon Sep 17 00:00:00 2001 From: Alexander Motin Date: Tue, 29 Jun 2021 08:59:14 -0400 Subject: [PATCH] Avoid 64bit division in multilist index functions The number of sublists in a multilist is relatively small. We dont need 64 bits to calculate an index. 32 bits is sufficient and makes the code more efficient. Reviewed-by: Matthew Ahrens Reviewed-by: Brian Behlendorf Reviewed-by: Mark Maybee Signed-off-by: Alexander Motin Sponsored-By: iXsystems, Inc. Closes #12288 --- module/zfs/arc.c | 5 +++-- module/zfs/dbuf.c | 5 +++-- module/zfs/dmu_objset.c | 10 +++++++++- module/zfs/metaslab.c | 7 ++++++- 4 files changed, 21 insertions(+), 6 deletions(-) diff --git a/module/zfs/arc.c b/module/zfs/arc.c index 7d892f4c7b..2226539a1e 100644 --- a/module/zfs/arc.c +++ b/module/zfs/arc.c @@ -7459,9 +7459,10 @@ arc_state_multilist_index_func(multilist_t *ml, void *obj) * Also, the low order bits of the hash value are thought to be * distributed evenly. Otherwise, in the case that the multilist * has a power of two number of sublists, each sublists' usage - * would not be evenly distributed. + * would not be evenly distributed. In this context full 64bit + * division would be a waste of time, so limit it to 32 bits. */ - return (buf_hash(hdr->b_spa, &hdr->b_dva, hdr->b_birth) % + return ((unsigned int)buf_hash(hdr->b_spa, &hdr->b_dva, hdr->b_birth) % multilist_get_num_sublists(ml)); } diff --git a/module/zfs/dbuf.c b/module/zfs/dbuf.c index 8e55a613bb..45886e5739 100644 --- a/module/zfs/dbuf.c +++ b/module/zfs/dbuf.c @@ -622,9 +622,10 @@ dbuf_cache_multilist_index_func(multilist_t *ml, void *obj) * Also, the low order bits of the hash value are thought to be * distributed evenly. Otherwise, in the case that the multilist * has a power of two number of sublists, each sublists' usage - * would not be evenly distributed. + * would not be evenly distributed. In this context full 64bit + * division would be a waste of time, so limit it to 32 bits. */ - return (dbuf_hash(db->db_objset, db->db.db_object, + return ((unsigned int)dbuf_hash(db->db_objset, db->db.db_object, db->db_level, db->db_blkid) % multilist_get_num_sublists(ml)); } diff --git a/module/zfs/dmu_objset.c b/module/zfs/dmu_objset.c index 22deee7f3d..af107fb8ad 100644 --- a/module/zfs/dmu_objset.c +++ b/module/zfs/dmu_objset.c @@ -399,7 +399,15 @@ static unsigned int dnode_multilist_index_func(multilist_t *ml, void *obj) { dnode_t *dn = obj; - return (dnode_hash(dn->dn_objset, dn->dn_object) % + + /* + * The low order bits of the hash value are thought to be + * distributed evenly. Otherwise, in the case that the multilist + * has a power of two number of sublists, each sublists' usage + * would not be evenly distributed. In this context full 64bit + * division would be a waste of time, so limit it to 32 bits. + */ + return ((unsigned int)dnode_hash(dn->dn_objset, dn->dn_object) % multilist_get_num_sublists(ml)); } diff --git a/module/zfs/metaslab.c b/module/zfs/metaslab.c index 0ddad5b026..08d7a56354 100644 --- a/module/zfs/metaslab.c +++ b/module/zfs/metaslab.c @@ -1874,7 +1874,12 @@ static unsigned int metaslab_idx_func(multilist_t *ml, void *arg) { metaslab_t *msp = arg; - return (msp->ms_id % multilist_get_num_sublists(ml)); + + /* + * ms_id values are allocated sequentially, so full 64bit + * division would be a waste of time, so limit it to 32 bits. + */ + return ((unsigned int)msp->ms_id % multilist_get_num_sublists(ml)); } uint64_t