Re: Parallel vacuum workers prevent the oldest xmin from advancing

On Thu, Nov 11, 2021 at 9:11 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
>
> Hi,
>
> On 2021-11-11 12:22:42 +0900, Masahiko Sawada wrote:
> > > 2.
> > > LWLockAcquire(ProcArrayLock, LW_SHARED);
> > >
> > > + flags = proc->statusFlags;
> > > +
> > > + /*
> > > + * If the source xact has any statusFlags, we re-grab ProcArrayLock
> > > + * on exclusive mode so we can copy it to MyProc->statusFlags.
> > > + */
> > > + if (flags != 0)
> > > + {
> > > + LWLockRelease(ProcArrayLock);
> > > + LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
> > > + }
> > >
> > >
> > > This looks a bit odd to me. It would have been better if we know when
> > > to acquire an exclusive lock without first acquiring the shared lock.
> >
> > I think we should acquire an exclusive lock only if status flags are
> > not empty. But to check the status flags we need to acquire a shared
> > lock. No?
>
> This seems like an unnecessary optimization. ProcArrayInstallRestoredXmin()
> only happens in the context of much more expensive operations.
>

Fair point. I think that will also make the change in
ProcArrayInstallRestoredXmin() appear neat.

> I think it might be worth asserting that the set of flags we're copying is a
> known subset of the flags that are valid to copy from the source.
>

Sounds reasonable.

--
With Regards,
Amit Kapila.