Re: Reducing ClogControlLock contention

On Tue, Jun 30, 2015 at 4:02 PM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:

> ClogControlLock contention is high at commit time. This appears to be due
> to the fact that ClogControlLock is acquired in Exclusive mode prior to
> marking commit, which then gets starved by backends running
> TransactionIdGetStatus().
>
> Proposal for improving this is to acquire the ClogControlLock in Shared
> mode, if possible.
>
> This is safe because people checking visibility of an xid must always run
> TransactionIdIsInProgress() first to avoid race conditions, which will
> always return true for the transaction we are currently committing. As a
> result, we never get concurrent access to the same bits in clog, which
> would require a barrier.
>
> Two concurrent writers might access the same word concurrently, so we
> protect against that with a new CommitLock. We could partition that by
> pageno also, if needed.
>

Could it be possible to see some performance numbers? For example with a
simple pgbench script doing a bunch of tiny transactions, with many
concurrent sessions (perhaps hundreds).
--
Michael