This document summarizes a presentation about using MongoDB Management Service (MMS) to monitor key metrics and improve MongoDB performance. It defines important MMS metrics like page faults per operation, CPU usage, locks and queues. It also discusses log file metrics like documents scanned, ordered scans and moved documents. The document provides guidelines for analyzing these metrics to determine if issues are CPU-bound, disk-bound, RAM-bound or related to other resources. It then gives recommendations for addressing different types of performance bottlenecks based on the metrics.

1. MongoDB Performance Tuning
with MMS
Presentation outline
By Enteros CTO
Ron Warshawsky
Ro@enteros.com
408-207-8408
Enteros, Inc.

2. MongoDb
2014-03-13 Enteros, Inc.
Overview
What Is MMS
MongoDB Management Service is a tool used to determine the health of a system
and identify the root cause of performance issues. We will segment this presentation
into two parts
Defining Key Metrics
Improving Performance

3. MongoDb
2014-03-13 Enteros, Inc.
Defining Key Metrics
We will define two types of metrics here:
MMS Metrics
Log File Metrics

4. MongoDb
2014-03-13 Enteros, Inc.
MMS Metrics
By examining these key metrics you can very quickly get a good picture of
what is going on inside a MongoDB system and what computing resources
(CPU, RAM, disk) are performance bottlenecks. Below is an outlook of the
MMS metrics.
PF/OP (Page
Faults/Opcounters)
CPU Time (IOWait and User)
Lock Percent and Queues

5. MongoDb
2014-03-13 Enteros, Inc.
MMS Metrics
PF/OP (Page Faults/Opcounters)
Between 5 and 10 page faults per second (left) compared to more than 4,000 operations per second
(right). A PF/OP of 0.001 (5 / 4000) is close enough to zero to classify as a low disk I/O requirement.
If PF/OP is…
near 0 – reads rarely require disk I/O
near 1 – reads regularly require disk I/O
greater than 1 – reads require heavy disk I/O

6. MongoDb
2014-03-13 Enteros, Inc.
MMS Metrics
CPU Time (IOWait and User)
CPU graphs from two different instances:
One experiencing high CPU IOWait (left) and the other experiencing high CPU User (right).
The CPU Time graph shows how the CPU cores are spending their
cycles. CPU IOWait reflects the fraction of time spent waiting for
the network or disk, while CPU User measures computation. Note
that to view CPU Time in MMS, you must also install munin.

7. MongoDb
2014-03-13 Enteros, Inc.
MMS Metrics
Lock Percent and Queues
Lock fluctuating with daily load (left) and corresponding queues (right)
Lock Percent and Queues tend to go hand in hand — the longer locking
operations take, the more other operations wait on them. The formation
of locks and queues isn’t necessarily cause for alarm in a healthy system,
but they are very good severity indicators when you and your app already
know things are slow.

8. MongoDb
2014-03-13 Enteros, Inc.
MMS Metrics
Lock Percent and Queues
Lock fluctuating with daily load (left) and corresponding queues (right)
The CPU Time graph shows how the CPU cores are spending their
cycles. CPU IOWait reflects the fraction of time spent waiting for
the network or disk, while CPU User measures computation. Note
that to view CPU Time in MMS, you must also install munin.

9. MongoDb
2014-03-13 Enteros, Inc.
Log File Metrics
In addition to key MMS metrics we also use MongoDB log files to discuss for
analysis.
nScanned
ScanAndOrder
nmoved

10. MongoDb
2014-03-13 Enteros, Inc.
Log File Metrics
In addition to key MMS metrics we also use MongoDB log files for analysis and
discuss here.
nScanned
ScanAndOrder
nmoved

11. MongoDb
2014-03-13 Enteros, Inc.
Log File Metrics
nScanned:
1. The number of objects the database examines to service a query.
This counts either documents or index entries, any of which may need to be obtained
from disk.
2. It is a good measure of the cost of an operation both in terms of I/O, object
manipulation, and memory use.
3. Ideally nscanned is not larger than nreturned, the number of results returned. Also
compare nscanned to the size of the collection. If nscanned is large, it is usually because
an index is not being used, or that the index was not selective enough.
4. Moving large numbers of documents into memory for these operations is a common
cause of page faults and CPU IOWait.

12. MongoDb
2014-03-13 Enteros, Inc.
Log File Metrics
scanAndOrder:
1. This is true when MongoDB performs an in-memory ordering operation to
satisfy the sort/orderby component of a query. For best performance, all
sort/orderby clauses should be satisfied by indexes. Sorting results at query time
can be inefficient (as compared with using an index that already contains
documents in the desired sort order), and can even block other operations if done
while holding the write lock, as with an update or findAndModify. Because sorting
requires computation, CPU User time can be expected.

13. MongoDb
2014-03-13 Enteros, Inc.
Log File Metrics
nmoved:
Indicates the number of documents that moved on disk during the operation. The
higher “nmoved”, the more intensive the operation. This is because when moving a
document: the new document location must be in memory, the document must be
copied, the old document’s location must be cleared, and index entries must be
updated to point to the new location.

14. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
Improving Performance
correctly determining whether one of these
metrics is high or low depends on their
values relative to each other. Here are some
rules of thumb.
• PF/OP is high if it is greater than 0.25
(indicates that your instance is page-
faulting for roughly 25% of operations)
CPU IOWait or CPU User is high if
• It is greater than 50% or
• It is more than twice as large as the next
highest CPU metric
There are eight possible overall
assessments given these three metrics.

15. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
Unbound
This means that either this database isn’t being used, or it is operating optimally.
Instead of relaxing, consider creating test data and a script to load-test your queries.

16. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
CPU-Bound
In this case, page faults are not severe enough to prompt any appreciable IOWait. As
long as the CPU isn’t idle, it’s somewhat expected that User time will appear the highest.
If locking and queueing are occurring, you’re probably seeing in-memory table scans and
scanAndOrder operations that are not bottlenecking due to an abundance of RAM or a
lack of load.
Run through the following steps:
• Check your logs for operations with high nscanned or scanAndOrder during periods
of high lock/queue, and index accordingly.
• Check your queries for CPU-intensive operators like $all, $push/$pop/$addToSet, as
well as updates to large documents, and especially updates to documents with large
arrays (or large subdocument arrays).
• Obtain more and/or faster CPU cores. Importantly, if your database is write-heavy,
keep in mind that only one CPU per database can write at a time (owing to that
thread holding the write lock).

17. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
Disk-Bound
if you’re experiencing trouble or if this sort of activity is coming in bursts, then getting the
most out of your current disk is about finding the source of I/OWait. So, even though the
ratio of PF/OP is low, we’re interested in actual Page Faults, which could still put a
noticeable drag on performance.
Let’s look at some sources of page faults:
• Page faults from insert/update operations – The write load to a database results in
page faults as fresh extents are brought into memory to handle new or moved
documents. Adding RAM will not alleviate this cost, so consider faster disks, like SSDs,
or sharding.
• Page faults from a shifting working set – These are the page faults that can be
prevented with additional RAM or with more efficient indexing.
• Page faults from MongoDB’s internal operations – These are few in number and
cannot be prevented.

18. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
Disk-Bound: Continued
If you’ve accounted for or the majority of your page faults, the remaining I/OWait in the
system should be coming from flushing the journal file and the background
synchronization of memory-mapped data to disk. Check writeToDataFilesMB on the
Journal Stats graph and the Background Flush Avg graph to see if spikes match the
elevated IOWait. Upgrade your disks and/or look into getting your journal on a separate
volume than your data. You can also separate databases using the directoryperdb mongod
option.
The last resort before sharding that youshould be verifying r data model is optimized for
writes. Check for collections with a large number of indexes, especially multi-key indexes,
and look for alternatives. Check your logs for the “nmoved” flag. If you find a lot of
documents are being moved during updates, examine the usePowerOf2Sizes setting for
those collections. When it comes to write-friendly data models, look for anything that
reduces the number of pages (index or data) that need to be modified when documents
are inserted or updated.

19. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
CPU/Disk-Bound
Treat elevated IOWait and User times as independently high, and consider each
of the previous two sections. We don’t see this combination a lot but it is
definitely possible, such as in the case of a high-RAM, write-heavy cluster that is
missing an index. If you are experiencing locks and queues, use them to guide the
time ranges you focus on.

20. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
RAM-Bound
If you are running high PF/OP without CPU IOWait, then:
• your operation counts are relatively low (indicating little to no operational activity).
• you have fantastic disks that are covering up for what would otherwise be a “RAM-Bound and
Disk-Bound” HHL case.
• you just started the database and are “warming” your cache. Wait until memory is fully utilized
and reassess.
If you are experiencing locks and queues, immediately evaluate your indexes to make sure missing
indexes are not prompting the need to page fault. Check your logs during periods of high page
faulting. Focus on operations with high nscanned values. Increase RAM if necessary, and you should
arrive at the coveted LLL case.
If you are not locking/queueing, this is technically a sustainable state, but be aware that any growth in
operation volume or data size–or even a barely-quantifiable shift in your working data set–could lead
to increasing IOWait, locks, and queues.

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2014-03-13 Enteros, Inc.
Improving Performance
RAM/CPU Bound
This scenario is similar to the RAM-Bound case above, except that high CPU User activity may be
covering up for what would otherwise be a high IOWait. Tackle the CPU portion first, then move
towards page faults, in the following steps:
1. Look for nscanned and scanAndOrder activity. On high-write clusters, consider the sheer number
of indexes in your write-collections. Maintaining indexes during inserts and updates can be
responsible for this, as can other CPU-intensive operators like $all, $push/$pop/$addToSet,
updates to large documents, and updates to documents with large arrays (or large subdocument
arrays).
2. Once the CPU User time has decreased, reconsider your case. If you have moved into HHL
territory, go to that section. If your CPU User time is still high, beef up your CPU if possible.
3. If your PF/OP is still high, be prepared to move to faster disk as soon as locks and queues begin to
occur.
4. Finally, consider sharding.

22. MongoDb
2014-03-13 Enteros, Inc.
Improving Performance
RAM/Disk Bound
This is possibly the most common problem case, and the solution is fairly straightforward
compared to the other cases we’ve dealt with. Your activity is disk-heavy. Index to reduce
nscanned, or add RAM either by scaling your machines vertically; or scaling horizontally via
sharding.

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2014-03-13 Enteros, Inc.
Improving Performance
BoundAllAround
The solution to this protracted situation depends on which of these three metrics is highest. In
general, we recommend tackling this as “RAM/Disk-Bound” HHL case first, because lack of RAM is
relatively easy to diagnose and many of the solutions for high CPU IOWait can contribute to CPU
User improvements.
So, check your logs for scanAndOrder and high nscanned numbers and then add indexes to reduce
them. Increase RAM if IOWait continues, and then re-evaluate your case. If you still end up here,
and if your locks and queues are still a problem, consider increasing your hardware capacity.
It could be premature to consider sharding straight from this highly protracted case. Ideally, attempt
a resolution that brings you to another case that we’ve explored, and consider sharding in that
context.

24. Enteros
2014-03-13 Enteros, Inc.
Upbeat High Load Capture
Database Root Cause and Spike Analysis for multi-tiered applications
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25. Enteros
2014-03-13 Enteros, Inc.
Upbeat High Load Capture
Supported Infrastructure, Database, Application server, OS monitoring
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26. MongoDb
2014-03-13 Enteros, Inc.
Enteros, Inc
http://www.enteros.com
Enteros is an innovative software company specializing in
Performance Management and Load Testing Software for
Production Databases - RDBMS and NOSQL/Big Data
Enteros solutions enable IT professionals to identify
and remediate performance problems in business-
critical databases with unprecedented speed, accuracy
and scope.
Ron Warshawsky; ron@enteros.com
408-207-8408