False Contention

For the last few weeks, I've been buried in PostgreSQL performance analysis, mostly focusing on the workload generated by "pgbench -S" under high concurrency.  In other words, lots and lots of very simple SELECT statements on a single table.  Such workloads can generate serious internal contention within PostgreSQL on systems within many CPU cores.

But it's interesting to note that most of the contention points I've so far identified are what might be called "false contention".  The transactions generated by this workload need to perform operations such as:

- acquire an AccessShareLock on the target relation and its index to guard against a concurrent drop or schema change
- acquire an ExclusiveLock on their VXID in case another transaction wishes to wait for transaction end
- read the list of pending "invalidation" events, which are generally created by DDL
- read the list of in-progress XIDs, to generate a snapshot
- find the root index block for the table's primary key index in the shared buffer pool
- momentarily pin the block containing the root index page into the buffer pool, so that it can't be evicted while we're examining it
- momentarily lock the root index page, so that no new items can be added until we've decided which downlink to follow

Now, in fact, in this workload, there is no concurrent DDL against the relevant table and index, or any other one; no one is attempting to wait for the completion of any VXID; the list of in-progress XIDs can easily be simultaneously read by many backends; and the root index page is in no danger either of being modified or of being evicted.  In short, the problem is not that there are resource conflicts, but that verifying that no resource conflicts exist is itself eating up too many resources.