Re: index prefetching

On Wed, Dec 20, 2023 at 7:11 AM Tomas Vondra
<tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>
I was going through to understand the idea, couple of observations

--
+ for (int i = 0; i < PREFETCH_LRU_SIZE; i++)
+ {
+ entry = &prefetch->prefetchCache[lru * PREFETCH_LRU_SIZE + i];
+
+ /* Is this the oldest prefetch request in this LRU? */
+ if (entry->request < oldestRequest)
+ {
+ oldestRequest = entry->request;
+ oldestIndex = i;
+ }
+
+ /*
+ * If the entry is unused (identified by request being set to 0),
+ * we're done. Notice the field is uint64, so empty entry is
+ * guaranteed to be the oldest one.
+ */
+ if (entry->request == 0)
+ continue;

If the 'entry->request == 0' then we should break instead of continue, right?

---
/*
* Used to detect sequential patterns (and disable prefetching).
*/
#define PREFETCH_QUEUE_HISTORY 8
#define PREFETCH_SEQ_PATTERN_BLOCKS 4

If for sequential patterns we search only 4 blocks then why we are
maintaining history for 8 blocks

---

+ *
+ * XXX Perhaps this should be tied to effective_io_concurrency somehow?
+ *
+ * XXX Could it be harmful that we read the queue backwards? Maybe memory
+ * prefetching works better for the forward direction?
+ */
+ for (int i = 1; i < PREFETCH_SEQ_PATTERN_BLOCKS; i++)

Correct, I think if we fetch this forward it will have an advantage
with memory prefetching.

--
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com