Re: Increase Vacuum ring buffer.

On Thu, Jul 20, 2017 at 7:59 AM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> >
> > Initially I wanted to make BAS_BULKWRITE and BAS_VACUUM ring sizes
> > configurable, but after testing I don't see much gain from increasing
> > ring buffer above 16MB. So I propose just 1 line change.
>
> I think the question for this patch is "so, why didn't we do it this
> way originally?".
>
> It's no secret that making the ring buffer larger will improve
> performance -- in fact, not having a ring buffer at all would improve
> performance even more. But it would also increase the likelihood that
> the background work of vacuum would impact the performance of
> foreground operations, which is already a pretty serious problem that
> we probably don't want to make worse.

But having a very fast sequence of fdatasync calls is not terribly friendly
to the performance of the foreground operations, either.

I think the reason we didn't do it this way originally is tied the same
reason that autovacuum_vacuum_cost_delay = 20ms by default. If we want it
to be heavily throttled, there isn't much point in using a larger ring
buffer. It is just wasted space. Maybe we could have it start out at
BAS_VACUUM's default size, then grow by one buffer every time it had to
issue a WAL sync, until it reached BAS_BULKWRITE's size where it would max
out.

Cheers,

Jeff