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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
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GPG Key ID: 4AEE18F83AFDEB23
3 changed files with 55 additions and 2 deletions
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1
include/sys/arc.h
View File

@ -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);

52
module/zfs/arc.c
View File

@ -448,9 +448,14 @@ typedef struct arc_stats {
* by multiple buffers.
*/
kstat_named_t arcstat_mutex_miss;
/*
* Number of buffers skipped when updating the access state due to the
* header having already been released after acquiring the hash lock.
*/
kstat_named_t arcstat_access_skip;
/*
* Number of buffers skipped because they have I/O in progress, are
* indrect prefetch buffers that have not lived long enough, or are
* indirect prefetch buffers that have not lived long enough, or are
* not from the spa we're trying to evict from.
*/
kstat_named_t arcstat_evict_skip;
@ -688,6 +693,7 @@ static arc_stats_t arc_stats = {
{ "mfu_ghost_hits", KSTAT_DATA_UINT64 },
{ "deleted", KSTAT_DATA_UINT64 },
{ "mutex_miss", KSTAT_DATA_UINT64 },
{ "access_skip", KSTAT_DATA_UINT64 },
{ "evict_skip", KSTAT_DATA_UINT64 },
{ "evict_not_enough", KSTAT_DATA_UINT64 },
{ "evict_l2_cached", KSTAT_DATA_UINT64 },
@ -5611,6 +5617,50 @@ arc_access(arc_buf_hdr_t *hdr, kmutex_t *hash_lock)
}
}
/*
* This routine is called by dbuf_hold() to update the arc_access() state
* which otherwise would be skipped for entries in the dbuf cache.
*/
void
arc_buf_access(arc_buf_t *buf)
{
mutex_enter(&buf->b_evict_lock);
arc_buf_hdr_t *hdr = buf->b_hdr;
/*
* Avoid taking the hash_lock when possible as an optimization.
* The header must be checked again under the hash_lock in order
* to handle the case where it is concurrently being released.
*/
if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
mutex_exit(&buf->b_evict_lock);
return;
}
kmutex_t *hash_lock = HDR_LOCK(hdr);
mutex_enter(hash_lock);
if (hdr->b_l1hdr.b_state == arc_anon || HDR_EMPTY(hdr)) {
mutex_exit(hash_lock);
mutex_exit(&buf->b_evict_lock);
ARCSTAT_BUMP(arcstat_access_skip);
return;
}
mutex_exit(&buf->b_evict_lock);
ASSERT(hdr->b_l1hdr.b_state == arc_mru ||
hdr->b_l1hdr.b_state == arc_mfu);
DTRACE_PROBE1(arc__hit, arc_buf_hdr_t *, hdr);
arc_access(hdr, hash_lock);
mutex_exit(hash_lock);
ARCSTAT_BUMP(arcstat_hits);
ARCSTAT_CONDSTAT(!HDR_PREFETCH(hdr) && !HDR_PRESCIENT_PREFETCH(hdr),
demand, prefetch, !HDR_ISTYPE_METADATA(hdr), data, metadata, hits);
}
/* a generic arc_read_done_func_t which you can use */
/* ARGSUSED */
void

4
module/zfs/dbuf.c
View File

@ -2821,8 +2821,10 @@ __dbuf_hold_impl(struct dbuf_hold_impl_data *dh)
return (SET_ERROR(ENOENT));
}
if (dh->dh_db->db_buf != NULL)
if (dh->dh_db->db_buf != NULL) {
arc_buf_access(dh->dh_db->db_buf);
ASSERT3P(dh->dh_db->db.db_data, ==, dh->dh_db->db_buf->b_data);
}
ASSERT(dh->dh_db->db_buf == NULL || arc_referenced(dh->dh_db->db_buf));