Using condition variables to wait for checkpoints

Hi,

On 2019-03-13 11:56:19 +1300, Thomas Munro wrote:

A user complained about CREATE DATABASE taking > 200ms even with fsync
set to off. Andres pointed out that that'd be the clunky poll/sleep
loops in checkpointer.c.

Here's a draft patch to use condition variables instead.

Unpatched:

postgres=# checkpoint;
CHECKPOINT
Time: 101.848 ms

Patched:

postgres=# checkpoint;
CHECKPOINT
Time: 1.851 ms

Neat. That's with tiny shmem though, I bet?

+        <row>
+         <entry><literal>CheckpointDone</literal></entry>
+         <entry>Waiting for a checkpoint to complete.</entry>
+        </row>

+        <row>
+         <entry><literal>CheckpointStart</literal></entry>
+         <entry>Waiting for a checkpoint to start.</entry>
+        </row>

Not sure I like these much, but I can't quite ome up with something
meaningfully better.

Looks good to me. Having useful infrastructure is sure cool.

- Andres

On Tue, Mar 12, 2019 at 7:12 PM Andres Freund <andres@anarazel.de> wrote:

Having useful infrastructure is sure cool.

Yay!

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

On Thu, Mar 14, 2019 at 1:15 AM Robert Haas <robertmhaas@gmail.com> wrote:

On Tue, Mar 12, 2019 at 7:12 PM Andres Freund <andres@anarazel.de> wrote:

Having useful infrastructure is sure cool.

Yay!

+1

I renamed the CVs because the names I had used before broke the
convention that variables named ckpt_* are protected by ckpt_lck, and
pushed.

There are some other things like this in the tree (grepping for
poll/pg_usleep loops finds examples in xlog.c, standby.c, ...). That
might be worth looking into.

--
Thomas Munro
https://enterprisedb.com

On Thu, Mar 14, 2019 at 11:05 AM Thomas Munro <thomas.munro@gmail.com> wrote:

I renamed the CVs because the names I had used before broke the
convention that variables named ckpt_* are protected by ckpt_lck, and
pushed.

Erm... this made successful checkpoints slightly faster but failed
checkpoints infinitely slower. It would help if we woke up CV waiters
in the error path too. Patch attached.

--
Thomas Munro
https://enterprisedb.com