Re: WAL prefetch

On 06/21/2018 04:01 PM, Konstantin Knizhnik wrote:
> I continue my experiments with WAL prefetch.
> I have embedded prefetch in Postgres: now walprefetcher is started
> together with startup process and is able to help it to speedup recovery.
> The patch is attached.
>
> Unfortunately result is negative (at least at my desktop: SSD, 16Gb
> RAM). Recovery with prefetch is 3 times slower than without it.
> What I am doing:
>
> Configuration:
>     max_wal_size=min_wal_size=10Gb,
>     shared)buffers = 1Gb
> Database:
>      pgbench -i -s 1000
> Test:
>      pgbench -c 10 -M prepared -N -T 100 -P 1
>      pkill postgres
>      echo 3 > /proc/sys/vm/drop_caches
>      time pg_ctl -t 1000 -D pgsql -l logfile start
>
> Without prefetch it is 19 seconds (recovered about 4Gb of WAL), with
> prefetch it is about one minute. About 400k blocks are prefetched.
> CPU usage is small (<20%), both processes as in "Ds" state.
>

Based on a quick test, my guess is that the patch is broken in several
ways. Firstly, with the patch attached (and wal_prefetch_enabled=on,
which I think is needed to enable the prefetch) I can't even restart the
server, because pg_ctl restart just hangs (the walprefetcher process
gets stuck in WaitForWAL, IIRC).

I have added an elog(LOG,...) to walprefetcher.c, right before the
FilePrefetch call, and (a) I don't see any actual prefetch calls during
recovery but (b) I do see the prefetch happening during the pgbench.
That seems a bit ... wrong?

Furthermore, you've added an extra

signal_child(BgWriterPID, SIGHUP);

to SIGHUP_handler, which seems like a bug too. I don't have time to
investigate/debug this further.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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