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Remove hash_elements_max accounting from DBUF and ARC
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Those values require global atomics to get current hash_elements
values in few of the hottest code paths, while in all the years I
never cared about it.  If somebody wants, it should be easy to
get it by periodic sampling, since neither ARC header nor DBUF
counts change so fast that it would be difficult to catch.

For now I've left hash_elements_max kstat for ARC, since it was
used/reported by arc_summary and it would break older versions,
but now it just reports the current value.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.
Closes openzfs#16759
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amotin authored and behlendorf committed Nov 21, 2024
1 parent 9206039 commit f7675ae
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Showing 4 changed files with 16 additions and 15 deletions.
5 changes: 1 addition & 4 deletions cmd/arc_summary
Original file line number Diff line number Diff line change
Expand Up @@ -662,10 +662,7 @@ def section_arc(kstats_dict):
print()

print('ARC hash breakdown:')
prt_i1('Elements max:', f_hits(arc_stats['hash_elements_max']))
prt_i2('Elements current:',
f_perc(arc_stats['hash_elements'], arc_stats['hash_elements_max']),
f_hits(arc_stats['hash_elements']))
prt_i1('Elements:', f_hits(arc_stats['hash_elements']))
prt_i1('Collisions:', f_hits(arc_stats['hash_collisions']))

prt_i1('Chain max:', f_hits(arc_stats['hash_chain_max']))
Expand Down
1 change: 1 addition & 0 deletions include/sys/arc_impl.h
Original file line number Diff line number Diff line change
Expand Up @@ -942,6 +942,7 @@ typedef struct arc_sums {
wmsum_t arcstat_evict_l2_eligible_mru;
wmsum_t arcstat_evict_l2_ineligible;
wmsum_t arcstat_evict_l2_skip;
wmsum_t arcstat_hash_elements;
wmsum_t arcstat_hash_collisions;
wmsum_t arcstat_hash_chains;
aggsum_t arcstat_size;
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13 changes: 7 additions & 6 deletions module/zfs/arc.c
Original file line number Diff line number Diff line change
Expand Up @@ -1074,12 +1074,9 @@ buf_hash_insert(arc_buf_hdr_t *hdr, kmutex_t **lockp)
ARCSTAT_BUMP(arcstat_hash_collisions);
if (i == 1)
ARCSTAT_BUMP(arcstat_hash_chains);

ARCSTAT_MAX(arcstat_hash_chain_max, i);
}
uint64_t he = atomic_inc_64_nv(
&arc_stats.arcstat_hash_elements.value.ui64);
ARCSTAT_MAX(arcstat_hash_elements_max, he);
ARCSTAT_BUMP(arcstat_hash_elements);

return (NULL);
}
Expand All @@ -1103,8 +1100,7 @@ buf_hash_remove(arc_buf_hdr_t *hdr)
arc_hdr_clear_flags(hdr, ARC_FLAG_IN_HASH_TABLE);

/* collect some hash table performance data */
atomic_dec_64(&arc_stats.arcstat_hash_elements.value.ui64);

ARCSTAT_BUMPDOWN(arcstat_hash_elements);
if (buf_hash_table.ht_table[idx] &&
buf_hash_table.ht_table[idx]->b_hash_next == NULL)
ARCSTAT_BUMPDOWN(arcstat_hash_chains);
Expand Down Expand Up @@ -7008,6 +7004,9 @@ arc_kstat_update(kstat_t *ksp, int rw)
wmsum_value(&arc_sums.arcstat_evict_l2_ineligible);
as->arcstat_evict_l2_skip.value.ui64 =
wmsum_value(&arc_sums.arcstat_evict_l2_skip);
as->arcstat_hash_elements.value.ui64 =
as->arcstat_hash_elements_max.value.ui64 =
wmsum_value(&arc_sums.arcstat_hash_elements);
as->arcstat_hash_collisions.value.ui64 =
wmsum_value(&arc_sums.arcstat_hash_collisions);
as->arcstat_hash_chains.value.ui64 =
Expand Down Expand Up @@ -7432,6 +7431,7 @@ arc_state_init(void)
wmsum_init(&arc_sums.arcstat_evict_l2_eligible_mru, 0);
wmsum_init(&arc_sums.arcstat_evict_l2_ineligible, 0);
wmsum_init(&arc_sums.arcstat_evict_l2_skip, 0);
wmsum_init(&arc_sums.arcstat_hash_elements, 0);
wmsum_init(&arc_sums.arcstat_hash_collisions, 0);
wmsum_init(&arc_sums.arcstat_hash_chains, 0);
aggsum_init(&arc_sums.arcstat_size, 0);
Expand Down Expand Up @@ -7590,6 +7590,7 @@ arc_state_fini(void)
wmsum_fini(&arc_sums.arcstat_evict_l2_eligible_mru);
wmsum_fini(&arc_sums.arcstat_evict_l2_ineligible);
wmsum_fini(&arc_sums.arcstat_evict_l2_skip);
wmsum_fini(&arc_sums.arcstat_hash_elements);
wmsum_fini(&arc_sums.arcstat_hash_collisions);
wmsum_fini(&arc_sums.arcstat_hash_chains);
aggsum_fini(&arc_sums.arcstat_size);
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12 changes: 7 additions & 5 deletions module/zfs/dbuf.c
Original file line number Diff line number Diff line change
Expand Up @@ -89,7 +89,6 @@ typedef struct dbuf_stats {
kstat_named_t hash_misses;
kstat_named_t hash_collisions;
kstat_named_t hash_elements;
kstat_named_t hash_elements_max;
/*
* Number of sublists containing more than one dbuf in the dbuf
* hash table. Keep track of the longest hash chain.
Expand Down Expand Up @@ -134,7 +133,6 @@ dbuf_stats_t dbuf_stats = {
{ "hash_misses", KSTAT_DATA_UINT64 },
{ "hash_collisions", KSTAT_DATA_UINT64 },
{ "hash_elements", KSTAT_DATA_UINT64 },
{ "hash_elements_max", KSTAT_DATA_UINT64 },
{ "hash_chains", KSTAT_DATA_UINT64 },
{ "hash_chain_max", KSTAT_DATA_UINT64 },
{ "hash_insert_race", KSTAT_DATA_UINT64 },
Expand All @@ -154,6 +152,7 @@ struct {
wmsum_t hash_hits;
wmsum_t hash_misses;
wmsum_t hash_collisions;
wmsum_t hash_elements;
wmsum_t hash_chains;
wmsum_t hash_insert_race;
wmsum_t metadata_cache_count;
Expand Down Expand Up @@ -432,8 +431,7 @@ dbuf_hash_insert(dmu_buf_impl_t *db)
db->db_hash_next = h->hash_table[idx];
h->hash_table[idx] = db;
mutex_exit(DBUF_HASH_MUTEX(h, idx));
uint64_t he = atomic_inc_64_nv(&dbuf_stats.hash_elements.value.ui64);
DBUF_STAT_MAX(hash_elements_max, he);
DBUF_STAT_BUMP(hash_elements);

return (NULL);
}
Expand Down Expand Up @@ -506,7 +504,7 @@ dbuf_hash_remove(dmu_buf_impl_t *db)
h->hash_table[idx]->db_hash_next == NULL)
DBUF_STAT_BUMPDOWN(hash_chains);
mutex_exit(DBUF_HASH_MUTEX(h, idx));
atomic_dec_64(&dbuf_stats.hash_elements.value.ui64);
DBUF_STAT_BUMPDOWN(hash_elements);
}

typedef enum {
Expand Down Expand Up @@ -903,6 +901,8 @@ dbuf_kstat_update(kstat_t *ksp, int rw)
wmsum_value(&dbuf_sums.hash_misses);
ds->hash_collisions.value.ui64 =
wmsum_value(&dbuf_sums.hash_collisions);
ds->hash_elements.value.ui64 =
wmsum_value(&dbuf_sums.hash_elements);
ds->hash_chains.value.ui64 =
wmsum_value(&dbuf_sums.hash_chains);
ds->hash_insert_race.value.ui64 =
Expand Down Expand Up @@ -1004,6 +1004,7 @@ dbuf_init(void)
wmsum_init(&dbuf_sums.hash_hits, 0);
wmsum_init(&dbuf_sums.hash_misses, 0);
wmsum_init(&dbuf_sums.hash_collisions, 0);
wmsum_init(&dbuf_sums.hash_elements, 0);
wmsum_init(&dbuf_sums.hash_chains, 0);
wmsum_init(&dbuf_sums.hash_insert_race, 0);
wmsum_init(&dbuf_sums.metadata_cache_count, 0);
Expand Down Expand Up @@ -1077,6 +1078,7 @@ dbuf_fini(void)
wmsum_fini(&dbuf_sums.hash_hits);
wmsum_fini(&dbuf_sums.hash_misses);
wmsum_fini(&dbuf_sums.hash_collisions);
wmsum_fini(&dbuf_sums.hash_elements);
wmsum_fini(&dbuf_sums.hash_chains);
wmsum_fini(&dbuf_sums.hash_insert_race);
wmsum_fini(&dbuf_sums.metadata_cache_count);
Expand Down

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