7975c5e0a9
Commit 4de82f7d7 increased the WAL flush rate, mainly to increase the likelihood that hint bits can be set quickly. More quickly set hint bits can reduce contention around the clog et al. But unfortunately the increased flush rate can have a significant negative performance impact, I have measured up to a factor of ~4. The reason for this slowdown is that if there are independent writes to the underlying devices, for example because shared buffers is a lot smaller than the hot data set, or because a checkpoint is ongoing, the fdatasync() calls force cache flushes to be emitted to the storage. This is achieved by flushing WAL only if the last flush was longer than wal_writer_delay ago, or if more than wal_writer_flush_after (new GUC) unflushed blocks are pending. Based on some tests the default for wal_writer_delay is 1MB, which seems to work well both on SSD and rotational media. To avoid negative performance impact due to 4de82f7d7 an earlier commit (db76b1e) made SetHintBits() more likely to succeed; preventing performance regressions in the pgbench tests I performed. Discussion: 20160118163908.GW10941@awork2.anarazel.de
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