Re: Slow standby snapshot

Hello, Andres.

Thanks for your feedback.

>> Maybe use a hashtable of running transactions? It will be slightly faster
>> when adding\removing single transactions. But much worse when doing
>> KnownAssignedXidsRemove().
> Why would it be worse for KnownAssignedXidsRemove()? Were you intending to
> write KnownAssignedXidsGet[AndSetXmin]()?

It is actually was a hashtable in 2010. It was replaced by Simon Riggs
in 2871b4618af1acc85665eec0912c48f8341504c4.

> I'm not sure that we need to care as much about the cost of
> KnownAssignedXidsGetAndSetXmin() - for one, the caching we now have makes that
> less frequent.

It is still about 5-7% of CPU for a typical workload, a considerable
amount for me. And a lot of systems still work on 12 and 13.
The proposed approach eliminates KnownAssignedXidsGetAndSetXmin from
the top by a small changeset.

> But more importantly, it'd not be hard to maintain an
> occasionally (or opportunistically) maintained denser version for
> GetSnapshotData() - there's only a single writer, making the concurrency
> issues a lot simpler.

Could you please explain it in more detail?
Single writer and GetSnapshotData() already exclusively hold
ProcArrayLock at the moment of KnownAssignedXidsRemove,
KnownAssignedXidsGetAndSetXmin, and sometimes KnownAssignedXidsAdd.

BTW, the tiny thing we could also fix now is (comment from code):

> (We could dispense with the spinlock if we were to
> * create suitable memory access barrier primitives and use those instead.)
> * The spinlock must be taken to read or write the head/tail pointers unless
> * the caller holds ProcArrayLock exclusively.

Thanks,
Michail.