stuff that matters, monitoring, tuning

1. Mongo performance
Stuff that matters
Monitoring
Tuning
Conclusion
vladimir.malyk@gmail.com

2. Mongo performance
Stuff that matters

3. Mongo Pain. Write lock
Multiple readers / Single-writer lock
( http://en.wikipedia.org/wiki/Readers-writer_lock)
ver < 2.0 - has a process-wide write lock
2.1.1+ per-database locking
( https://jira.mongodb.org/browse/SERVER-4328 )
per-collection locking
( https://jira.mongodb.org/browse/SERVER-1240 )

4. Mongo Pain. mmap
http://docs.mongodb.org/manual/faq/storage/
Page faults: will occur if you’re attempting to access part
of a memory-mapped file that isn’t in memory.
( http://docs.mongodb.org/manual/reference/glossary/#term-page-fault )
Performance: page fault can take around 40,000 times
longer than a nonfaulting memory operation
ver 2.0+, this is addressed by detecting the likelihood of a page fault and
releasing the lock before faulting - YIELD operation;
SSD is a doctor's stuff in this case;

5. Mongo Atomicity
Write operations are atomic on the level of a single
document: no single write operation can atomically affect more
than one document or more than one collection.
(http://docs.mongodb.org/manual/tutorial/isolate-sequence-of-operations/)
hint: isolates a write operation that affects multiple documents
from other write operations
(http://docs.mongodb.org/manual/reference/operator/isolated/)
http://docs.mongodb.org/manual/reference/command/findAndModify/

6. Mongo performance
Monitoring

7. Monitoring: HTTP Console
http://docs.mongodb.org/ecosystem/tools/http-interf
aces/#http-console
http://dbhost.net:28017/
Replica Set Admin UI, Oplog status
clients
DBTOP
write lock % time in write lock, by 4 sec periods (form ~12% to
~6%)
Log - By default the slow operation threshold is 100 millis

8. Monitoring: SSH Console
netstat -n | wc -l
ps aux | grep mongodb
cat /proc/{pid}/limits | grep "Max open files"
ulimit -n 65000
(http://www.andrewrollins.com/2010/10/20/mongodb-open-file-limit/)

9. Monitoring: Custom
http://sj.malyk/admin/MongoProfileLog/
milestone marks: do fake update before and after app execution
http://docs.mongodb.org/manual/tutorial/manage-the-database-profi
ler/
http://docs.mongodb.org/manual/reference/database-profiler/
https://gist.github.com/vladimir-malyk/5047107
https://gist.github.com/kgorman/995a3aa5b35e92e5ab57

10. Mongo performance
Tuning

11. Tuning: Heavy tricks
degradation - don't write to an unimportant
collections;
partitioning - move an unimportant
collections to a separate replicaset.

12. Tuning: Indexes
* Create Indexes to Support Your Queries
* Use Compound Indexes to Support Several Different Queries
* -10% write performance for each additional index
(http://www.slideshare.net/mongodb/mongodb-performance-tuning)
* Create Indexes that Support Covered Queries
* Use Indexes to Sort Query Results
* http://blog.mongolab.com/2012/06/cardinal-ins/
* First, fields on which you will query for exact values.
* Second, fields on which you will sort.
* Finally, fields on which you will query for a range of val.
* Ensure Indexes Fit RAM
* Creating index on a big collection takes a lot of time with write
lock

13. Tuning: Common tricks
http://blog.serverdensity.com/mongodb-schema-design-pitfalls/
* Avoid growing documents (“moved” in system.profile)
* Use field modifiers: Instead of sending a whole new document
to update an existing one, you can set or remove specific fields
* Preallocate documents: preallocate the document with
placeholder values, then use the $set field modifier to change the
actual value later
* Field names take up space:
http://blog.serverdensity.com/on-shortened-field-names-in-mongod
b/
* Consider using _id for your own purposes

14. Tuning: Explain
http://docs.mongodb.org/manual/reference/explain/
* http://docs.mongodb.org/manual/reference/explain/#explain.cursor
* http://docs.mongodb.org/manual/reference/explain/#explain.net
* http://docs.mongodb.org/manual/reference/explain/#explain.nscanned
* http://docs.mongodb.org/manual/reference/explain/#explain.n
* http://docs.mongodb.org/manual/reference/explain/#explain.scanAndOrder
* http://docs.mongodb.org/manual/reference/explain/#explain.indexOnly
* http://docs.mongodb.org/manual/reference/explain/#explain.nYields
* http://docs.mongodb.org/manual/reference/explain/#explain.allPlans

15. Mongo performance
Conclusion

16. Mongo: Tips and tricks
* deal with db disk fragmentation:
* manual defragmentation (via replica resync);
* http://docs.mongodb.org/manual/reference/command/compact/
* Can't take a write lock while out of disk space:
* you need some free space - db defrag will help you;

17. Mongo: Concurrency
http://docs.mongodb.org/manual/faq/concurrency/
http://docs.mongodb.org/manual/faq/concurrency/#how-do-i-see-the-status-
of-locks-on-my-mongod-instances
http://docs.mongodb.org/manual/faq/concurrency/#which-operations-lock-th
e-database
http://docs.mongodb.org/manual/faq/concurrency/#which-administrative-co
mmands-lock-the-database
* db.auth() - lock the database but only hold the lock for a very short time
http://blog.serverdensity.com/goodbye-global-lock-mongodb-2-0-vs-2-2/
why it's a good idea to upgrade your mongodb

18. Mongo: Some links
http://www.slideshare.net/mongodb/mongodb-performance-tuning
http://blog.pythonisito.com/2011/12/mongodbs-write-lock.html