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 <[email protected]> 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/ >
