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Re: LMDB Hanging Problem in Multi-Threaded Application

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






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