Re: Speed up Clog Access by increasing CLOG buffers

On Tue, Jul 4, 2017 at 12:33 AM, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> wrote:
> I have updated the patch to support wait events and moved it to upcoming CF.

This patch doesn't apply any more, but I made it apply with a hammer
and then did a little benchmarking (scylla, EDB server, Intel Xeon
E5-2695 v3 @ 2.30GHz, 2 sockets, 14 cores/socket, 2 threads/core).
The results were not impressive. There's basically no clog contention
to remove, so the patch just doesn't really do anything. For example,
here's a wait event profile with master and using Ashutosh's test
script with 5 savepoints:

1 Lock | tuple
2 IO | SLRUSync
5 LWLock | wal_insert
5 LWLock | XidGenLock
9 IO | DataFileRead
12 LWLock | lock_manager
16 IO | SLRURead
20 LWLock | CLogControlLock
97 LWLock | buffer_content
216 Lock | transactionid
237 LWLock | ProcArrayLock
1238 IPC | ProcArrayGroupUpdate
2266 Client | ClientRead

This is just a 5-minute test; maybe things would change if we ran it
for longer, but if only 0.5% of the samples are blocked on
CLogControlLock without the patch, obviously the patch can't help
much. I did some other experiments too, but I won't bother
summarizing the results here because they're basically boring. I
guess I should have used a bigger machine.

Given that we've changed the approach here somewhat, I think we need
to validate that we're still seeing a substantial reduction in
CLogControlLock contention on big machines.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company