Re: SSI implementation question

On Wed, Oct 19, 2011 at 12:56:58PM -0400, Tom Lane wrote:
> Is it really necessary for GetSerializableTransactionSnapshotInt to
> acquire an empty SERIALIZABLEXACT before it acquires a snapshot?
> If so, why? 

*That* isn't necessary, no. It is necessary, however, to acquire the
snapshot while SerializableXactHashLock is held. There are a couple
reasons for this: the sxact's lastCommitBeforeSnapshot needs to match
the snapshot, SxactGlobalXmin needs to be set to the correct value,
etc. That's why the call to GetSnapshotData happens from where it does

> The proposed synchronized-snapshots feature will mean
> that the allegedly-new snapshot actually was taken some time before,
> so it seems to me that either this is not necessary or we cannot use
> a synchronized snapshot in a serializable xact.

There are definitely potential problems here. If the original snapshot
doesn't belong to an active serializable transaction, we may have
discarded the state we need to do SSI, e.g. we might have already
cleaned up SIREAD locks from concurrent committed transactions.

I assume the answer here is going to have to be to either refuse to
start a serializable transaction if that's the case, or make saving a
snapshot inhibit some of the SSI cleanup.

> In the same vein, why is it necessary to be holding
> SerializableXactHashLock (exclusively, yet) while acquiring the
> snapshot?  That seems rather bad from a concurrency standpoint, and
> again it's going to be pretty meaningless if we're just installing a
> pre-existing snapshot.

Yes, it's bad. I'm working on a design to address
SerializableXactHashLock contention, but there needs to be some locking
here for the reasons I mentioned above. I think the best we can do here
is to acquire a lock in shared mode when registering a serializable
transaction and in exclusive mode when committing. (Which is what you'd
expect, I guess; it's the same story as ProcArrayLock, and for most of
the same reasons.) Obviously, we'll also want to minimize the amount of
work we're doing while holding that lock.

Dan

-- 
Dan R. K. Ports              MIT CSAIL                http://drkp.net/