1. Concurrency Internals in v2.2
Dwight Merriman, CEO & Co-Founder, 10gen
This session begins at 2:45PM...
#mongosf

2. Version #s
vA.B.C
Even B’s are stable
Odd B’s are dev branch.
Current stable = 2.0.x (e.g. v2.0.4)
Current dev branch = 2.1.x (e.g. v2.1.1)
(not for production)
Once blessed, 2.1.x becomes v2.2.0
Thus
1.6 -> 1.8
1.8 -> 2.0
2.0 -> 2.2
2.2 -> 2.4
are similar magnitudes of change

3. Two big changes in 2.2
1. Eliminate the global reader/writer lock
“Granularization”
First step is db-level locks.
2. PageFaultException architecture
(yield lock on a page fault)

4. Lock per db
• Reader/writer lock per database
– First step towards more granularity
• Global lock is still possible but now very rare (e.g.
journaling locks the world for short intervals; the
journal is shared by all dbs)
• More granularity should be fairly easy to add in the
future – hard part is done
– Were places in code where assumptions were made
about exclusivity when in a write lock; those are gone
now

5. Lock per db
• In v2.1.1+ sometimes two db’s are locked
by the same thread
– admin db for security info lookup
– local db for replication (writing to the oplog)

6. Structure of a write operation
Lock(mydb);
Do_write_operation(mydb); // slow
Lock(local_database);
Write_to_replication_oplog(); // fast
Unlock(local_database);
Unlock(mydb);

7. Lock status can be inspected
• currentOp()
• serverStatus()
• MMS monitoring will evolve too to help
with instrumentation of system status in
this regard

8. PageFaultException
• Improves concurrency within a single
collection

9. PageFaultException
http://blog.pythonisito.com/2011/12/mongodbs-write-lock.html

10. PageFaultException
• If on a write operation
– Mutation has yet to occur
– And we are going to page fault
• Then
– Throw PageFaultException
– While unlocked, touch the page
– Retry the operation

11. The Future
• More granularity
• Now that there is some, much easier
• Collection level for sure
• Document level locking (latching) – there
are some nuances (btrees) – but at least
directionally, yes

12. Thanks
Please help us test the upcoming v2.2 release by hammering the dev branch releases in your
QA environment (i.e., v2.1.1).
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