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Add DB_RF_NOPREFETCH to dbuf_read()s in dnode.c

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Prefetching of dnodes in dbuf_read() can cause significant mutex 
contention for some workloads and isn't very helpful.  This is  
because we already get 32 dnodes for each block read, and when 
iterating over a directory we prefetch the dnodes in the directory.
Disable this prefetching to prevent the lock contention.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Submitted-by: Adam Moss <c@yotes.com>
Submitted-by: Matthew Ahrens <mahrens@delphix.com>
Signed-off-by: Adam Moss <c@yotes.com>
Closes #10877 
Closes #10953
This commit is contained in:
Adam D. Moss 2020-09-25 13:49:22 -07:00 committed by GitHub
parent 2e407941a2
commit acfd2d4641
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1 changed files with 4 additions and 2 deletions
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6
module/zfs/dnode.c
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@ -1355,7 +1355,8 @@ dnode_hold_impl(objset_t *os, uint64_t object, int flag, int slots,
* We do not need to decrypt to read the dnode so it doesn't matter
* if we get the encrypted or decrypted version.
*/
err = dbuf_read(db, NULL, DB_RF_CANFAIL | DB_RF_NO_DECRYPT);
err = dbuf_read(db, NULL, DB_RF_CANFAIL |
DB_RF_NO_DECRYPT | DB_RF_NOPREFETCH);
if (err) {
DNODE_STAT_BUMP(dnode_hold_dbuf_read);
dbuf_rele(db, FTAG);
@ -2396,7 +2397,8 @@ dnode_next_offset_level(dnode_t *dn, int flags, uint64_t *offset,
return (SET_ERROR(ESRCH));
}
error = dbuf_read(db, NULL,
DB_RF_CANFAIL | DB_RF_HAVESTRUCT | DB_RF_NO_DECRYPT);
DB_RF_CANFAIL | DB_RF_HAVESTRUCT |
DB_RF_NO_DECRYPT | DB_RF_NOPREFETCH);
if (error) {
dbuf_rele(db, FTAG);
return (error);