Re: Maybe some more low-hanging fruit in the latestCompletedXid patch.

Is this a TODO?

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Florian G. Pflug wrote:
> Tom Lane wrote:
> > "Florian G. Pflug" <fgp(at)phlo(dot)org> writes:
> >> Currently, we do not assume that either the childXids array, nor the xid
> >> cache in the proc array are sorted by ascending xid order. I believe that
> >> we could simplify the code, further reduce the locking requirements, and
> >> enabled a transaction to de-overflow it's xid cache if we assume that those
> >> arrays are in ascending xid order.
> >
> > "de-overflowing" the cache sounds completely unsafe, as other backends need
> > that state to determine whether they need to look into pg_subtrans.
>
> We'd only de-overflow if we abort *all* xids that are missing from the
> xid cache. And only after marking them as aborted in the clog. If someone
> concurrently checks for an overflow, and already sees the new (non-overflowed)
> state, than he'll assume the xid is not running if he hasn't found it in
> the array. Which is correct - we just aborted it.
>
> Plus, removing the exclusive lock doesn't depend on de-overflowing. It's
> just something that seems rather easy to do once the subxid handling is
> in a state that allows concurrent removal of entries. If it turns out that
> it's not that easy, than I'll just drop the idea again.
>
> > I still don't believe you can avoid taking exclusive lock, either; your
> > argument here did not address latestCompletedXid.
>
> Sorry, not addressing latestCompletedXid was an oversight :-(.
> My point is the we only *need* to advance latestCompletedXid on COMMITS. We do
> so for aborts only to avoid running with unnecessarily low xmins after
> a transaction ABORT. That corner case can only happen after a toplevel
> ABORT, though - aborting subxacts cannot change the xmin, because the
> toplevel xact will have a lower xid than any of it's subtransactions anyway.
>
> We can therefore just remember the largest assigned xid for a given transaction,
> and update latestCompletedXid to that on toplevel commit or abort. That
> prevents that corner-case too, without updating latestCompletedXid during
> subxact abort.
>
> > But the main point remains this: there is no evidence whatsoever that these
> > code paths are sufficiently performance-critical to be worth speeding up by
> > making the code more fragile.
>
> The gain will be less than that of the locking improvements done so far.
> It will be a win for heavy users of plpgsql BEGIN/END/EXCEPTION blocks,
> though I think.
>
> We'll also save some cycles in TransactionIdIsInProgress, because we can
> use a binary search, but that's just an added bonus.
>
> I'm currently trying to code up a patch, since it's easier to judge the
> correctness of actual code than that of a mere proposals. I'll do some
> benchmarking when the patch is done to see if it brings measurable benefits.
>
> greetings, Florian Pflug
>
>
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Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://postgres.enterprisedb.com

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