Re: Flush some statistics within running transactions

I took a look at 0001 in depth.

> I don't think this feature could add a noticeable performance impact, so the tests
> have been that simple. Do you think we should worry more?

One observation is there's no coordination between ANYTIME and
TXN_BOUNDARY flushes. While PGSTAT_MIN_INTERVAL
prevents a backend from flushing more than once per second, a backend can
still perform both an ANYTIME flush and a TXN_BOUNDARY flush within
the same 1-second window. Not saying this will be a real problem in
the real-world,
but we definitely took measures in the current implementation to avoid
this scenario.

A few other comments on 0001

+ /* Skip if completely idle */
+ if (!DoingCommandRead || IsTransactionOrTransactionBlock())
+ pgstat_report_anytime_stat(false);

Does this need to be conditional? worst case, we return right away with an empty
list. Best case, is we are consistently flushing.

+ /*
+ * When in anytime_only mode, the list may not be empty because
+ * FLUSH_AT_TXN_BOUNDARY entries were skipped.
+ */
+ Assert(!anytime_only || dlist_is_empty(&pgStatPending) ==
!have_pending);

Checking for !anytime_only is unnecessary here.
"list_is_empty(&pgStatPending) == !have_pending"
should be true regardless of ANYTIME or TXN_BOUNDARY, right?

Below are a couple of edits for comments I felt would improve
readability of the code.

1/
/*
- * Flush non-transactional stats
- *
- * This is safe to call even inside a transaction. It only flushes stats
- * kinds marked as FLUSH_ANYTIME.
- *
- * This allows long running transactions to report activity without waiting
- * for transaction to finish.
+ * Flushes only FLUSH_ANYTIME stats using non-blocking locks. Transactional
+ * stats (FLUSH_AT_TXN_BOUNDARY) remain pending until transaction boundary.
+ * Safe to call inside transactions.
*/

2/
typedef enum PgStat_FlushBehavior
{
- FLUSH_ANYTIME, /* All fields can
flush anytime */
- FLUSH_AT_TXN_BOUNDARY, /* All fields need transaction
boundary */
+ FLUSH_ANYTIME, /* All fields can be
flushed anytime,
+ *
including within transactions */
+ FLUSH_AT_TXN_BOUNDARY, /* All fields can only be flushed at
+ *
transaction boundary */
} PgStat_FlushBehavior;

I will start looking at the remaining patches next.

--
Sami Imseih
Amazon Web Services (AWS)