Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation()

On Fri, Mar 9, 2018 at 8:49 PM, Peter Eisentraut <
peter(dot)eisentraut(at)2ndquadrant(dot)com> wrote:

> On 2/1/18 19:21, Simon Riggs wrote:
> > If we really can't persuade you of that, it doesn't sink the patch. We
> > can have the WAL pointer itself - it wouldn't save space but it would
> > at least alleviate the spinlock.
>
> Do you want to send in an alternative patch that preserves the WAL
> pointer and only changes the locking?
>
>
Sorry for the delay. Here is an updated patch which now replaces xl_prev
with xl_curr, thus providing similar safeguards against corrupted or torn
WAL pages, but still providing benefits of atomic operations.

I repeated the same set of tests and the results are almost similar. These
tests are done on a different AWS instance though and hence not comparable
to previous tests. What we do in these tests is essentially
call ReserveXLogInsertLocation() 1M times to reserve 256 bytes each time,
in a single select statement (the function is exported and a SQL-callable
routine is added for the purpose of the tests)

Patched:
======
#clients #tps
1 34.195311
2 29.001584
4 31.712009
8 35.489272
16 41.950044

Master:
======
#clients #tps
1 24.128004
2 12.326135
4 8.334143
8 16.035699
16 8.502794

So that's pretty good improvement across the spectrum.

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services