Re: suppressing useless wakeups in logical/worker.c

From: Nathan Bossart <nathandbossart(at)gmail(dot)com>
To: Amit Kapila <amit(dot)kapila16(at)gmail(dot)com>
Cc: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Thomas Munro <thomas(dot)munro(at)gmail(dot)com>, "Hayato Kuroda (Fujitsu)" <kuroda(dot)hayato(at)fujitsu(dot)com>, "pgsql-hackers(at)postgresql(dot)org" <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: suppressing useless wakeups in logical/worker.c
Date: 2023-02-01 00:05:21
Message-ID: 20230201000521.GA3147405@nathanxps13
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On Sat, Jan 28, 2023 at 10:26:25AM +0530, Amit Kapila wrote:
> On Fri, Jan 27, 2023 at 4:07 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> Returning to the prior patch ... I don't much care for this:
>> + /* Maybe there will be a free slot in a second... */
>> + retry_time = TimestampTzPlusSeconds(now, 1);
>> + LogRepWorkerUpdateSyncStartWakeup(retry_time);
>> We're not moving the goalposts very far on unnecessary wakeups if
>> we have to do that. Do we need to get a wakeup on sync slot free?
>> Although having to send that to every worker seems ugly. Maybe this
>> is being done in the wrong place and we need to find a way to get
>> the launcher to handle it.

It might be feasible to set up a before_shmem_exit() callback that wakes up
the apply worker (like is already done for the launcher). I think the
apply worker is ordinarily notified via the tablesync worker's notify_pid,
but AFAICT there's no guarantee that the apply worker hasn't restarted with
a different PID.

> + /*
> + * Since process_syncing_tables() is called conditionally, the
> + * tablesync worker start wakeup time might be in the past, and we
> + * can't know for sure when it will be updated again. Rather than
> + * spinning in a tight loop in this case, bump this wakeup time by
> + * a second.
> + */
> + now = GetCurrentTimestamp();
> + if (wakeup[LRW_WAKEUP_SYNC_START] < now)
> TimestampTzPlusSeconds(wakeup[LRW_WAKEUP_SYNC_START], 1);
> Do we see unnecessary wakeups without this, or delay in sync?

I haven't looked too cloesly to see whether busy loops are likely in

> BTW, do we need to do something about wakeups in
> wait_for_relation_state_change()?

... and wait_for_worker_state_change(), and copy_read_data(). From a quick
glance, it looks like fixing these would be a more invasive change. TBH
I'm beginning to wonder whether all this is really worth it to prevent
waking up once per second.

Nathan Bossart
Amazon Web Services:

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