This document discusses various aspects of MongoDB performance. It covers monitoring MongoDB performance through the HTTP console, SSH console, and custom monitoring. It also discusses tuning MongoDB performance through optimizing indexes, avoiding growing documents, preallocating documents, and using explain queries. The conclusion provides some tips on dealing with disk fragmentation, concurrency, and why it's good to upgrade MongoDB versions.

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