Hi, working on the checkpoint sorting/flushing patch I noticed a number of preexisting issues:
1) The progress passed to CheckpointWriteDelay() will often be wrong - it's calculated as num_written / num_to_write, but num_written is only incremented if the buffer hasn't since independently been written out. That's bad because it mean's we'll think we're further and further behind if there's independent writeout activity. Simple enough to fix, we gotta split num_written into num_written (for stats purposes) and num_processed (for progress). This is pretty much a bug, but I'm a slightly worried about backpatching a fix because it can have a rather noticeable behavioural impact. 2) CheckpointWriteDelay()'s handling of config file changes et al is pretty weird: The config is reloaded during a checkpoing iff it's not an immediate checkpoint and we're on schedule. I see very little justification for having the got_SIGHUP block inside that if block. 3) The static pg_usleep(100000L); in CheckpointWriteDelay() is a bad idea. On a system with low write activity (< 10 writes sec) this will lead to checkpoints finishing too early as there'll always be around ~10 writes a second. On slow IO, say a sdcard, that's bad. On system with a very high write throughput (say 1k+ buffers/sec) the unconditional 100ms sleep while on schedule will mean we're far behind after the sleep. This leads to rather bursty IO, and makes it more likely to run into dirty buffer limits and such. Just reducing the sleep from 100ms to 10ms leads to significant latency improvements. On pgbench rate limited to 5k tps: before: number of transactions skipped: 70352 (1.408 %) number of transactions above the 100.0 ms latency limit: 2817 (0.056 %) latency average: 5.266 ms latency stddev: 11.723 ms rate limit schedule lag: avg 3.138 (max 0.000) ms after: number of transactions skipped: 41696 (0.834 %) number of transactions above the 100.0 ms latency limit: 1736 (0.035 %) latency average: 4.929 ms latency stddev: 8.929 ms rate limit schedule lag: avg 2.835 (max 0.000) ms I think the sleep time should be computed adaptively based on the number of buffers remaining and the remaining time. There's probably better formulations, but that seems like an easy enough improvement and considerably better than now. 4) It's a bit dubious to only pgstat_send_bgwriter() when on schedule. Greetings, Andres Freund -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
