On Tue, Jun 30, 2015 at 4:02 PM, Simon Riggs <si...@2ndquadrant.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
