On 08/30/2012 10:14 AM, Marius Gedminas wrote:
On Wed, Aug 29, 2012 at 06:30:50AM -0400, Jim Fulton wrote:
On Wed, Aug 29, 2012 at 2:29 AM, Marius Gedminas <mar...@gedmin.as> wrote:
On Tue, Aug 28, 2012 at 06:31:05PM +0200, Vincent Pelletier wrote:
On Tue, 28 Aug 2012 16:31:20 +0200,
Martijn Pieters <m...@zopatista.com> wrote :
Anything else different? Did you make any performance comparisons
between RelStorage and NEO?

I believe the main difference compared to all other ZODB Storage
implementation is the finer-grained locking scheme: in all storage
implementations I know, there is a database-level lock during the
entire second phase of 2PC, whereas in NEO transactions are serialised
only when they alter a common set of objects.

This could be a compelling point.  I've seen deadlocks in an app that
tried to use both ZEO and PostgreSQL via the Storm ORM.  (The thread
holding the ZEO commit lock was blocked waiting for the PostgreSQL
commit to finish, while the PostgreSQL server was waiting for some other
transaction to either commit or abort -- and that other transaction
couldn't proceed because it was waiting for the ZEO lock.)

This sounds like an application/transaction configuration problem.


Here's the code to reproduce it: http://pastie.org/4617132

To avoid this sort of deadlock, you need to always commit in a
a consistent order.  You also need to configure ZEO (or NEO)
to time-out transactions that take too long to finish the second phase.

The deadlock happens in tpc_begin() in both threads, which is the first
phase, AFAIU.

AFAICS Thread #2 first performs tpc_begin() for ClientStorage and takes
the ZEO commit lock.  Then it enters tpc_begin() for Storm's
StoreDataManager and blocks waiting for a response from PostgreSQL --
which is delayed because the PostgreSQL server is waiting to see if
the other thread, Thread #1, will commit or abort _its_ transaction, which
is conflicting with the one from Thread #2.

Meanwhile Thread #1 is blocked in ZODB's tpc_begin(), trying to acquire the
ZEO commit lock held by Thread #2.

So thread 1 acquires in this order:

1. PostgreSQL
2. ZEO

Thread 2 acquires in this order:

1. ZEO
2. PostgreSQL

SQL databases handle deadlocks by detecting and automatically rolling back transactions, while the "transaction" package expects all data managers to completely avoid deadlocks using the sortKey method.

I haven't looked at the code, but I imagine Storm's StoreDataManager implements IDataManager. I wonder if StoreDataManager provides a consistent sortKey. The sortKey method must return a string (not an integer or other object) that is consistent yet different from all other participating data managers.


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