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commit df7b7bf64eb354114e6c519e3e03ffc446afa8ba
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Author: Nathan Scott <nathans@redhat.com>
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Date: Fri Nov 26 09:17:23 2021 +1100
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libpcp_pmda: add indom cache fast-paths for inst lookup beyond max
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We encountered a situation where indom cache loading consumed vast
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CPU resources for an indom of size ~150k instances. Profiling was
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used to identify the insert loop that ensures the inst linked list
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within the cache hash tables is sorted - this loop is O(N*2) as we
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potentially walk this list from the start on every insert during a
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cache load. Because cache loading happens from a sorted file, the
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worst-case scenario happened every time - each new instance insert
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occurs beyond the current maximum. Fortunately we maintain a last
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entry pointer, so the new fast path uses that first and falls back
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to the original behaviour for an out-of-order insertion.
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A second opportunity for the same optimization was identified when
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auditing the rest of cache.c - in the find_inst() routine for inst
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identifier lookups beyond the current maximum observed instance.
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Resolves Red Hat BZ #2024648
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diff --git a/src/libpcp_pmda/src/cache.c b/src/libpcp_pmda/src/cache.c
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index 0e66506d74..196ffc1da9 100644
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--- a/src/libpcp_pmda/src/cache.c
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+++ b/src/libpcp_pmda/src/cache.c
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@@ -328,6 +328,9 @@ find_inst(hdr_t *h, int inst)
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{
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entry_t *e;
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+ if ((e = h->last) != NULL && e->inst < inst)
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+ return NULL;
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+
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for (e = h->first; e != NULL; e = e->next) {
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if (e->inst == inst && e->state != PMDA_CACHE_EMPTY)
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break;
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@@ -621,7 +624,11 @@ insert_cache(hdr_t *h, const char *name, int inst, int *sts)
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*sts = PM_ERR_INST;
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return e;
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}
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- for (e = h->first; e != NULL; e = e->next) {
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+ /* if this entry is beyond the (sorted) list end, avoid linear scan */
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+ if ((e = h->last) == NULL || e->inst > inst)
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+ e = h->first;
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+ /* linear search over linked list, starting at either first or last */
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+ for (; e != NULL; e = e->next) {
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if (e->inst < inst)
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last_e = e;
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else if (e->inst > inst)
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