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Avoid 64bit division in multilist index functions

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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 <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> 
Reviewed-by: Mark Maybee <mark.maybee@delphix.com>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored-By: iXsystems, Inc.
Closes #12288
This commit is contained in:
Alexander Motin
2021-06-29 08:59:14 -04:00
committed by GitHub
parent f20fb199e6
commit 5b7053a9a5
4 changed files with 21 additions and 6 deletions
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module/zfs/dmu_objset.c
+9 -1
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@@ -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));
}