Re: pgbench vs. wait events
| От | Jeff Janes |
|---|---|
| Тема | Re: pgbench vs. wait events |
| Дата | |
| Msg-id | CAMkU=1zevoSVzaEtU7pjpk=tLg2bkM6v9imKRFrRaYwLK0-D=A@mail.gmail.com обсуждение исходный текст |
| Ответ на | Re: pgbench vs. wait events (Robert Haas <robertmhaas@gmail.com>) |
| Список | pgsql-hackers |
On Fri, Oct 7, 2016 at 1:28 PM, Robert Haas <robertmhaas@gmail.com> wrote:
On Fri, Oct 7, 2016 at 11:51 AM, Jeff Janes <jeff.janes@gmail.com> wrote:
> What happens if you turn fsync off? Once a xlog file is fully written, it
> is immediately fsynced, even if the backend is holding WALWriteLock or
> wal_insert (or both) at the time, and even if synchrounous_commit is off.
> Assuming this machine has a BBU so that it doesn't have to wait for disk
> rotation, still fsyncs are expensive because the kernel has to find all the
> data and get it sent over to the BBU, while holding locks.
Scale factor 300, 32 clients, fsync=off:
5 Lock | tuple
18 LWLockTranche | lock_manager
24 LWLockNamed | WALWriteLock
88 LWLockTranche | buffer_content
265 LWLockTranche | wal_insert
373 LWLockNamed | CLogControlLock
496 LWLockNamed | ProcArrayLock
532 Lock | extend
540 LWLockNamed | XidGenLock
545 Lock | transactionid
27067 Client | ClientRead
85364 |
Did the TPS go up appreciably?
But I'm not sure you're right about the way the fsync=off code works.
I think pg_fsync()/pg_fsync_writethrough()/pg_fsync_no_ writethrough()
look at enableFsync and just do nothing if it's false.
I think we are in agreement. I don't know which part you think is wrong. When I said immediately, I didn't mean unconditionally.
Anyway, based on the reduced wait events, I think this shows that if we need to do something in the xlog area, probably what it would be is to add a queue of fully written but un-synced xlog files, so that the syncing can be delegated to the background wal writer process. And of course anyone needing to actually flush their xlog would have to start by flushing the queue.
(Or perhaps just make the xlog files bigger, and call it a day)
Cheers,
Jeff
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