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Re: SSI patch version 14

From: Dan Ports <drkp(at)csail(dot)mit(dot)edu>
To: Jeff Davis <pgsql(at)j-davis(dot)com>
Cc: Kevin Grittner <Kevin(dot)Grittner(at)wicourts(dot)gov>,pgsql-hackers(at)postgresql(dot)org
Subject: Re: SSI patch version 14
Date: 2011-01-25 10:57:42
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Lists: pgsql-hackers
Thanks for working your way through this patch. I'm certainly well
aware that that's not a trivial task!

I'm suffering through a bout of insomnia, so I'll respond to some of
your high-level comments in hopes that serializability will help put me
to sleep (as it often does). I'll leave the more detailed code comments
for later when I'm actually looking at the code, or better yet Kevin
will take care of them and I won't have to. ;-)

On Tue, Jan 25, 2011 at 01:07:39AM -0800, Jeff Davis wrote:
> At a high level, there is a nice conceptual simplicity. Let me try to
> summarize it as I understand it:
>   * RW dependencies are detected using predicate locking.
>   * RW dependencies are tracked from the reading transaction (as an
>     "out") conflict; and from the writing transaction (as an "in"
>     conflict).
>   * Before committing a transaction, then by looking only at the RW 
>     dependencies (and predicate locks) for current and past 
>     transactions, you can determine if committing this transaction will
>     result in a cycle (and therefore a serialization anomaly); and if
>     so, abort it.

This summary is right on. I would add one additional detail or
clarification to the last point, which is that rather than checking for
a cycle, we're checking for a transaction with both "in" and "out"
conflicts, which every cycle must contain.

> That's where the simplicity ends, however ;)


> Tracking of RW conflicts of current and past transactions is more
> complex. Obviously, it doesn't keep _all_ past transactions, but only
> ones that overlap with a currently-running transaction. It does all of
> this management using SHMQueue. There isn't much of an attempt to
> gracefully handle OOM here as far as I can tell, it just throws an error
> if there's not enough room to track a new transaction (which is
> reasonable, considering that it should be quite rare and can be
> mitigated by increasing max_connections). 

If the OOM condition you're referring to is the same one from the
following comment, then it can't happen: (Apologies if I've
misunderstood what you're referring to.)

> * In RegisterSerializableTransactionInt, if it doesn't get an sxact, it
> goes into summarization. But summarization assumes that it has at least
> one finished xact. Is that always true? If you have enough memory to
> hold a transaction for each connection, plus max_prepared_xacts, plus
> one, I think that's true. But maybe that could be made more clear?

Yes -- the SerializableXact pool is allocated up front and it
definitely has to be bigger than the number of possible active
transactions. In fact, it's much larger: 10 * (MaxBackends +
max_prepared_xacts) to allow some room for the committed transactions
we still have to track.

> * In RegisterSerializableTransactionInt(), for a RO xact, it considers
> any concurrent RW xact a possible conflict. It seems like it would be
> possible to know whether a RW transaction may have overlapped with any
> committed RW transaction (in finishedLink queue), and only add those as
> potential conflicts. Would that work? If so, that would make more
> snapshots safe.

Interesting idea. That's worth some careful thought. I think it's
related to the condition that the RW xact needs to commit with a
conflict out to a transaction earlier than the RO xact. My first guess
is that this wouldn't make more transactions safe, but could detect
safe snapshots faster.

> * When a transaction finishes, then PID should probably be set to zero.
> You only use it for waking up a deferrable RO xact waiting for a
> snapshot, right?

Correct. It probably wouldn't hurt to clear that field when releasing
the transaction, but we don't use it after.

> * I'm a little unclear on summarization and writing to the SLRU. I don't
> see where it's determining that the predicate locks can be safely
> released. Couldn't the oldest transaction still have relevant predicate
> locks?

When a SerializableXact gets summarized to the SLRU, its predicate
locks aren't released; they're transferred to the dummy

> I'll keep working on this patch. I hope I can be of some help getting
> this committed, because I'm looking forward to this feature. And I
> certainly think that you and Dan have applied the amount of planning,
> documentation, and careful implementation necessary for a feature like
> this.

Hopefully my comments here will help clarify the patch. It's not lost
on me that there's no shortage of complexity in the patch, so if you
found anything particularly confusing we should probably add some
documentation to README-SSI.


Dan R. K. Ports              MIT CSAIL      

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