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Fix ARC hit rate

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When the compressed ARC feature was added in commit d3c2ae1
the method of reference counting in the ARC was modified.  As
part of this accounting change the arc_buf_add_ref() function
was removed entirely.

This would have be fine but the arc_buf_add_ref() function
served a second undocumented purpose of updating the ARC access
information when taking a hold on a dbuf.  Without this logic
in place a cached dbuf would not migrate its associated
arc_buf_hdr_t to the MFU list.  This would negatively impact
the ARC hit rate, particularly on systems with a small ARC.

This change reinstates the missing call to arc_access() from
dbuf_hold() by implementing a new arc_buf_access() function.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Tim Chase <tim@chase2k.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6171 
Closes #6852 
Closes #6989
This commit is contained in:
Brian Behlendorf
2018-01-08 09:52:36 -08:00
committed by GitHub
parent 390d679acd
commit 0873bb6337
3 changed files with 55 additions and 2 deletions
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include/sys/arc.h
+1
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@@ -263,6 +263,7 @@ void arc_buf_destroy(arc_buf_t *buf, void *tag);
void arc_buf_info(arc_buf_t *buf, arc_buf_info_t *abi, int state_index);
uint64_t arc_buf_size(arc_buf_t *buf);
uint64_t arc_buf_lsize(arc_buf_t *buf);
void arc_buf_access(arc_buf_t *buf);
void arc_release(arc_buf_t *buf, void *tag);
int arc_released(arc_buf_t *buf);
void arc_buf_sigsegv(int sig, siginfo_t *si, void *unused);