On 12/31/2013 09:18 AM, Peter Geoghegan wrote:
On Sun, Dec 29, 2013 at 9:09 AM, Heikki Linnakangas
While mulling this over further, I had an idea about this: suppose we
marked the tuple in some fashion that indicates that it's a promise
tuple. I imagine an infomask bit, although the concept makes me wince
a bit since we don't exactly have bit space coming out of our ears
there. Leaving that aside for the moment, whenever somebody looks at
the tuple with a mind to calling XactLockTableWait(), they can see
that it's a promise tuple and decide to wait on some other heavyweight
lock instead. The simplest thing might be for us to acquire a
heavyweight lock on the promise tuple before making index entries for
it, and then have callers wait on that instead always instead of
transitioning from the tuple lock to the xact lock.
Yeah, that seems like it should work. You might not even need an infomask
bit for that; just take the "other heavyweight lock" always before calling
XactLockTableWait(), whether it's a promise tuple or not. If it's not,
acquiring the extra lock is a waste of time but if you're going to sleep
anyway, the overhead of one extra lock acquisition hardly matters.
Are you suggesting that I lock the tuple only (say, through a special
LockPromiseTuple() call), or lock the tuple *and* call
XactLockTableWait() afterwards? You and Robert don't seem to be in
agreement about which here.
I meant the latter, ie. grab the new kind of lock first, then check if
the tuple is still there, and then call XactLockTableWait() as usual.
The inserter has to acquire the heavyweight lock before releasing the buffer
lock, because otherwise another inserter (or deleter or updater) might see
the tuple, acquire the heavyweight lock, and fall to sleep on
XactLockTableWait(), before the inserter has grabbed the heavyweight lock.
If that race condition happens, you have the original problem again, ie. the
updater unnecessarily waits for the inserting transaction to finish, even
though it already killed the tuple it inserted.
Right. Can you suggest a workaround to the above problems?
Umm, I did, in the next paragraph ;-) :
That seems easy to avoid. If the heavyweight lock uses the
transaction id as the key, just like
XactLockTableInsert/XactLockTableWait, you can acquire it before
doing the insertion.
Let me elaborate that. The idea is to have new heavy-weight lock that's
just like the transaction lock used by
XactLockTableInsert/XactLockTableWait, but separate from that. Let's
call it PromiseTupleInsertionLock. The insertion procedure in INSERT ...
ON DUPLICATE looks like this:
1. PromiseTupleInsertionLockAcquire(<my xid>)
2. Insert heap tuple
3. Insert index tuples
4. Check if conflict happened. Kill the already-inserted tuple on conflict.
5. PromiseTupleInsertionLockRelease(<my xid>)
IOW, the only change to the current patch is that you acquire the new
kind of lock before starting the insertion, and you release it after
you've killed the tuple, or you know you're not going to kill it.
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