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/