I realize that this isn't terribly critical, but I'd like to suggest
that commit_delay be made PGC_SIGHUP on 9.3 (it's currently
PGC_USERSET). It's not that a poorly chosen commit_delay setting has
the potential to adversely affect other backends where the setting
*has* been set in those other backends in a suitable way - the same
thing can surely be said for work_mem. It just seems to me that
commit_delay is now something that's intended to work at the cluster
granularity, and as such it seems like almost a misrepresentation to
make it PGC_USERSET.
The fact is that whichever backend happens to end up becoming the
group commit leader from one XLogFlush() call to the next is, for all
practical purposes, unpredictable. You cannot reasonably hope to avoid
a delay within an important transaction that needs to prioritize
keeping its own latency low over total cluster throughput. If you set
commit_delay to 0 in your important transaction with this is mind,
your chances of becoming the group commit leader and avoiding the
delay are slim to almost none. Much more often than not, the important
transaction will end up becoming a group commit follower, and it'll
still spend a significant fraction of commit_delay (about 1/2, on
average) blocking on LWLockAcquireOrWait().
--
Peter Geoghegan