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Optimize commit_siblings in two ways to improve group commit.

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First, avoid scanning the whole ProcArray once we know there
are at least commit_siblings active; second, skip the check
altogether if commit_siblings = 0.

Greg Smith
This commit is contained in:
Simon Riggs
2010-12-08 18:48:03 +00:00
parent 5a031a5556
commit e620ee35b2
5 changed files with 27 additions and 13 deletions
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17
doc/src/sgml/config.sgml
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@ -1683,17 +1683,24 @@ SET ENABLE_SEQSCAN TO OFF;
</indexterm>
<listitem>
<para>
Time delay between writing a commit record to the WAL buffer
and flushing the buffer out to disk, in microseconds. A
nonzero delay can allow multiple transactions to be committed
with only one <function>fsync()</function> system call, if
When the commit data for a transaction is flushed to disk, any
additional commits ready at that time are also flushed out.
<varname>commit_delay</varname> adds a time delay, set in
microseconds, before writing some commit records to the WAL
buffer and flushing the buffer out to disks. A nonzero delay
can allow more transactions to be committed with only one call
to the active <varname>wal_sync_method</varname>, if
system load is high enough that additional transactions become
ready to commit within the given interval. But the delay is
just wasted if no other transactions become ready to
commit. Therefore, the delay is only performed if at least
<varname>commit_siblings</varname> other transactions are
active at the instant that a server process has written its
commit record. The default is zero (no delay).
commit record. The default is zero (no delay). Since
all pending commit data flushes are written at every flush
regardless of this setting, it is rare that adding delay to
that by increasing this parameter will actually improve commit
performance.
</para>
</listitem>
</varlistentry>