Re: LMDB Hanging Problem in Multi-Threaded Application

[Howard Chu <hyc@symas.com>]

Eskandar Ensafi wrote:
> 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.

No. The documentation states EXACTLY: "a transaction must only be used by a 
single thread."

It does *NOT* say "one thread at a time."

Read more carefully.

> 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/


--
  -- Howard Chu
  CTO, Symas Corp.           http://www.symas.com
  Director, Highland Sun     http://highlandsun.com/hyc/
  Chief Architect, OpenLDAP  http://www.openldap.org/project/