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Re: LMDB Hanging Problem in Multi-Threaded Application

Thank you for your reply.  The documentation is not clear.  It states:

"A transaction and its cursors must only be used by a single thread, and a thread may only have a single transaction at a time."

When you protect the transaction with a mutex, only one thread uses it at a time, and only a single write transaction exists for the entire process.

Thanks,

Alex


> On Jun 8, 2016, at 2:45 PM, Howard Chu <hyc@symas.com> wrote:
> 
> Yahoo! wrote:
>> Hello,
>> 
>> I successfully ran a multi-threaded application using LMDB 0.9.10-1 on a
> 64-bit Ubuntu 14.04 machine with 4x10-core Xeon E5-2660 v3 CPUs and 132 GB
> RAM. I then attempted to run the same application on a CentOS 6.7 box with
> 2x4-core i7-4790K CPUs and 32 GB RAM, where I tried two versions of LMDB:
> first EPEL RPM package version 0.9.18-1, and then the latest version 0.9.70
> which I downloaded and compiled from GitHub sources on 4/27/2016. On the
> CentOS system with fewer cores, my application started to hang. I have traced
> the problem to a periodic commit, after which a new transaction must be
> created. I commit periodically to avoid a long delay when the application
> shuts down, at which point a 2 GB database is flushed to disk.
>> 
>> I have a single write transaction, which is protected by a mutex when
> calling mdb_txn_commit(), mdb_txn_begin() and mdb_put(). The threads are
> created by TBB using parallel_for, and each thread has access to the single
> MDB_dbi and associated MDB_txn for writing. When a counter hits a certain
> value, we lock a mutex and call mdb_txn_commit(), and then we create a new
> MDB_txn to replace the invalidated one. Other threads try to acquire the same
> mutex before attempting to call mdb_put(). After a new write transaction is
> created, the next call the mdb_put() works fine on my 40-core Ubuntu box and
> the database is created successfully, but the application hangs on my 8-core
> CentOS box. My only guess is that I am encountering a race condition that is
> somehow avoided on the faster machine.
>> 
>> Is the above valid LMDB usage?
> 
> No.
> 
>> I have tried to find information about
> multi-threaded writing, and I have not found any definitive answers.
> 
> http://lmdb.tech/doc/group__mdb.html#gad7ea55da06b77513609efebd44b26920
> 
> I don't know where you've been looking, but it's plainly stated in the docs.
> 
>> I would
> be happy to share the relevant code, but I need to first strip out a great
> deal of project-specific ancillary code, which will take some time.
> 
>> Thanks,
>> 
>> Alex
> 
> 
> -- 
>  -- Howard Chu
>  CTO, Symas Corp.           http://www.symas.com
>  Director, Highland Sun     http://highlandsun.com/hyc/
>  Chief Architect, OpenLDAP  http://www.openldap.org/project/
>