Thanks, this is great..
I have now learned that MDB_NOLOCK (with correct locking) allows me to get read-write transactions to cross thread boundaries (since they don't use TLS). I didn't see anywhere in the docs that read-write transactions don't use TLS, so I appreciate your input.
For read-only transactions, MDB_NOTLS and thread-crossing has been clear in the docs for a long time. So that part was easier. From what now I see, LMDB works great with M:N applications for both reading **and writing**. (M:N is still out there.. Erlang, Go, etc.)
Jun 8, 2023, 16:04 by hyc@symas.com:
Sam Dave wrote:
Very nice, I missed MDB_NOLOCK completely. This might work for me.
However, one thing got my curious about MDB_NOLOCK docs: "For proper operation [of MDB_NOLOCK] the caller must ... ensure that no readers are using old transactions while a writer is active."
Is this also true without MDB_NOLOCK, i.e. does the built-in locking mechanism also "ensure that no readers are using old transactions while a writer is active."? (I guess not because it sounds like it would break "readers don't block writers.")
In the built in mechanism, readers never block writers. When using your own locking, readers must block writers otherwise you'll corrupt the views that read txns are seeing.
Needless to say, with NOLOCK you must know what you're doing. From the level of this conversation so far, I wouldn't recommend you go this route.
Your initial problem seems to be that you don't understand the actual semantics of transactions and threads. LMDB expects them to be 1:1. That's it. Anything else is a waste, just like M:N threading (multiple user-level threads on top of a different number of kernel threads) is a waste and has been abandoned by everyone.
I was hoping I could do my own concurrency management only for write transactions without worrying about read transaction - I thought that's what "multiversioning" was all about.
Jun 7, 2023, 19:17 by hyc@symas.com:
Sam Dave wrote:
Read-write txns don't use TLS. TLS is only used for read-only txns.
Why then is it important that a read-write transaction stays on the same thread (including the final abort or commit)? (I just assumed it was because it needed access to TLS, looks like I was wrong)
Write txns are serialized by a mutex. The mutex is taken when a write txn begins and is released when the txn ends. It is illegal for any thread other than the original locking thread to release the mutex.
If you don't want these semantics, just use MDB_NOLOCK and do your own concurrency management.
Jun 7, 2023, 17:23 by hyc@symas.com:
Sam Dave wrote:
Alternatively, could a more stateful API be provided (as an alternative to API relying on TLS)?
Roughly what I have in mind is, mdb_txn_create() returns a state pointer, which should be passed into the next mdb_put(), which returns a state pointer, which should be passed into the next mdb_put(), and so on, until the final state is passed into mdb_commit() or mdb_abort().
A txn pointer is a state pointer.
Jun 7, 2023, 10:26 by samdave@tuta.io:
Thanks, I was just hoping abort was a (currently undocumented) exception to this :)
Would it be possible/feasible in the future to bring MDB_NOTLS support to read-write transactions? (or at least some partial NOTLS support for only e.g. abort)
Read-write txns don't use TLS. TLS is only used for read-only txns.
Motivation: Ability to abort "later" (e.g. at garbage collection in some languages).. as this "later" stuff does not necessarily happen on the original thread.
- Samuel
Jun 6, 2023, 18:44 by hyc@symas.com:
Sam Dave wrote:
Hi,
The documentation for mdb_txn_begin() says "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.".
Does this also hold for mdb_txn_abort() for read-write transactions? I.e. does the abort also have to happen on the same OS thread as all the previous things (begin, put, etc.)?
Calling mdb_txn_abort() requires passing the txn as a parameter, which looks pretty suspiciously to me like "using" it. Is this really a serious question?
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