On Thu, Nov 11, 2021 at 3:07 PM Amit Kapila <amit.kapil...@gmail.com> wrote: > > On Thu, Nov 11, 2021 at 10:40 AM Masahiko Sawada <sawada.m...@gmail.com> > wrote: > > > > On Thu, Nov 11, 2021 at 12:53 PM Amit Kapila <amit.kapil...@gmail.com> > > wrote: > > > > > > On Thu, Nov 11, 2021 at 9:11 AM Andres Freund <and...@anarazel.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. > > > > Agreed. > > > > This makes me think that it'd be better to copy status flags in a > > separate function rather than ProcArrayInstallRestoredXmin(). The > > current patch makes use of the fact that ProcArrayInstallRestoedXmin() > > acquires a shared lock in order to check the source's status flags. > > But if we can acquire an exclusive lock unconditionally in this > > context, it’s clearer to do in a separate function. > > > > Do you mean to say that do it in a separate function and call > immediately after StartParallelWorkerTransaction or do you mean to do > it in a separate function and invoke it from > ProcArrayInstallRestoedXmin()?
I meant the former. > I think the disadvantage I see by not > doing in ProcArrayInstallRestoedXmin is that we need to take procarray > lock twice (once in exclusive mode and then in shared mode) so doing > it in ProcArrayInstallRestoedXmin is beneficial from that angle. Right. I thought that this overhead is also negligible in this context. If that’s right, it’d be better to do it in a separate function from the clearness point of view. Also if we raise the lock level in ProcArrayInstallRestoredXmin(), a caller of the function who wants just to set xmin will end up acquiring an exclusive lock. Which is unnecessary for the caller. Regards, -- Masahiko Sawada EDB: https://www.enterprisedb.com/