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Improve management of statement timeouts.

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Commit f8e5f156b added private state in postgres.c to track whether
a statement timeout is running.  This seems like bad design to me;
timeout.c's private state should be the single source of truth about
that.  We already fixed one bug associated with failure to keep those
states in sync (cf. be42015fc), and I've got little faith that we
won't find more in future.  So get rid of postgres.c's local variable
by exposing a way to ask timeout.c whether a timeout is running.
(Obviously, such an inquiry is subject to race conditions, but it
seems fine for the purpose at hand.)

To make get_timeout_active() as cheap as possible, add a flag in
the per-timeout struct showing whether that timeout is active.
This allows some small savings elsewhere in timeout.c, mainly
elimination of unnecessary searches of the active_timeouts array.

While at it, fix enable_statement_timeout to not call disable_timeout
when statement_timeout is 0 and the timeout is not running.  This
avoids a useless deschedule-and-reschedule-timeouts cycle, which
represents a significant savings (at least one kernel call) when
there is any other active timeout.  Right now, there usually isn't,
but there are proposals around to change that.

Discussion: https://postgr.es/m/16035-456e6e69ebfd4374@postgresql.org
This commit is contained in:
Tom Lane
2019-10-25 11:41:16 -04:00
parent 2b2bacdca0
commit 22f6f2c1cc
3 changed files with 39 additions and 33 deletions
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22
src/backend/tcop/postgres.c
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@ -145,11 +145,6 @@ static bool DoingCommandRead = false;
static bool doing_extended_query_message = false;
static bool ignore_till_sync = false;
/*
* Flag to keep track of whether statement timeout timer is active.
*/
static bool stmt_timeout_active = false;
/*
* If an unnamed prepared statement exists, it's stored here.
* We keep it separate from the hashtable kept by commands/prepare.c
@ -4029,7 +4024,6 @@ PostgresMain(int argc, char *argv[],
*/
disable_all_timeouts(false);
QueryCancelPending = false; /* second to avoid race condition */
stmt_timeout_active = false;
/* Not reading from the client anymore. */
DoingCommandRead = false;
@ -4711,14 +4705,14 @@ enable_statement_timeout(void)
if (StatementTimeout > 0)
{
if (!stmt_timeout_active)
{
if (!get_timeout_active(STATEMENT_TIMEOUT))
enable_timeout_after(STATEMENT_TIMEOUT, StatementTimeout);
stmt_timeout_active = true;
}
}
else
disable_timeout(STATEMENT_TIMEOUT, false);
{
if (get_timeout_active(STATEMENT_TIMEOUT))
disable_timeout(STATEMENT_TIMEOUT, false);
}
}
/*
@ -4727,10 +4721,6 @@ enable_statement_timeout(void)
static void
disable_statement_timeout(void)
{
if (stmt_timeout_active)
{
if (get_timeout_active(STATEMENT_TIMEOUT))
disable_timeout(STATEMENT_TIMEOUT, false);
stmt_timeout_active = false;
}
}