AOBD 07/08 H. Galhardas

DBMS Performance Monitoring

Performance Monitoring Goals

Monitoring should check that the performance-influencing database parameters are correctly set and if they are not, it should point to where the problems are

Alternatively, you can sit tight and wait for the inevitable angry users’ complaints

OR

Monitoring(preventive

method)

Correct problemswhen detected

(reactive method)

How does one monitor a DBMS?

By extracting relevant performance indicators, (counters, gauges and details of internal DBMS’s activities)

By comparing the obtained values of these indicators against ideal values

But… there are many indicators!

A Consumer-Producer Chain of a DBMS’s Resources

High LevelConsumers

IntermediateResources/Consumers

PrimaryResources

PARSEROPTIMIZERPARSER

OPTIMIZER

EXECUTIONSUBSYSTEM

EXECUTIONSUBSYSTEM DISK

SYBSYSTEM DISK

SYBSYSTEM

CACHEMANAGERCACHE

MANAGER

LOGGINGSUBSYSTEMLOGGING

SUBSYSTEM

LOCKINGSUBSYSTEM LOCKING

SUBSYSTEM

NETWORKDISK/

CONTROLLERCPUMEMORY

sql commands

probingspotsfor

indicators

Recurrent Patterns of Problems

An overloading high-level consumer Ex: query accessing too many

rows A poorly parameterized

subsystem Ex: disk subsystem that stores

tables, indexes, logging in a single disk

An overloaded primary resource Ex: CPU busy because non-DB

processes are using it at the same time

Effects are not always felt first where the cause is!

A Systematic Approach to Monitoring

Question 1: Are critical queries being served in the most efficient manner?

Question 2: Are DBMS subsystems making optimal use of resources?

Question 3: Are there enough primary resources available and are they configured adequately, given the current and expected workload?

Extract indicators to answer the following questions

Investigating High Level Consumers: Critical query monitoring

Find criticalqueries

Found any?Investigatelower levels

Answer Q1over them

Overcon-sumption?

Tune problematicqueries

yes

yesno

no

Investigating Intermediate and Primary Resources: routine monitoring

Answer Q3

Problems at

OS level?Answer Q2

Tune low-levelresources

yesno

Problematic

subsystems?Tune

subsystemsInvestigateupper level

yes no

Tools used to gather information Event monitors Query plan explainers Performance monitors

Investigating High Level Consumers

Answer question 1: “Are critical queries being served in the most efficient manner?”

1. Identify the critical queries

2. Analyze their access plans

3. Profile their execution

Identifying Critical Queries

Critical queries are usually those that: Take a long time

Consume a great amount of resources (read a lot of data, perform heavy aggregation or sorting, lock an entire table, etc)

Are frequently executed Have to be answered in a short time

Often, a user complaint will tip us off.

Event Monitors to Identify Critical Queries

If no user complains... Record DBMS’s performance measurements, but only

when a system event happens Ex: new user connection, or beginning/end of the

execution of an SQL statement Logs the event time, duration and associated

performance indicator values Less overhead than other type of tools because

indicators are usually by-product of operations monitored and are accessed in a convenient time Particularly appropriate for capturing extraordinary

conditions, ex: deadlocks. Typical measures include CPU used, IO used, locks

obtained etc.

What should be measured and evaluated Interested in the end-of-statement or end-of-

transaction event Carry relevant info that indicate the resources the

executed query consumed: SQL of the statement, duration of execution, CPU, IO, cache,lock consumption statistics

Sort the collected data over a given resource consumption statistic and we can find the most expensive queries by that criterion

An example Event Monitor

• Several CPU indicatorssorted by Oracle’sTrace Data Viewer

• Similar tools: DB2’s Event Monitor and MSSQL’s Server Profiler

What to do in case of problems? Make sure expensive queries are tuned in the

best possible way May have to drill down through the

consumption chain to search for the real cause of the problem

Investigating High Level Consumers

Answer question 1: “Are critical queries being served in the most efficient manner?”

1. Identify the critical queries

2. Analyze their access plans

3. Profile their execution

Diagnose Expensive Queries: analyzing access plans

SQL commands are translated into an internal executable format before they are processed

After parsing, the optimizer enumerates and estimates the cost of a number of possible ways to execute that query

The best choice, according to the existing statistics, is chosen But this choice may not be the best one!

sqlsql parse rewriteenumerate/cost plans

generatechosen plan

planplan

PARSER/OPTIMIZER

Query Plan Explainers to Analyze Plans

Explainers usually depict an access plan as a (graphical) single-rooted tree in which sources at the leaf end are tables or indexes, and internal nodes are operators

These form an assembly line (actually tree) of tuples!

Most explainers will let you see the estimated costs for CPU consumption, I/O, and cardinality of each operator. If you see something strange, change an index.

An example Plan Explainer

• Access plan according to MSSQL’s Query Analyzer

• Similar tools: DB2’s Visual Explain and Oracle’s SQL AnalyzeTool

Finding Strangeness in Access PlansWhat to pay attention to on a plan

Access paths for each table For every table involved in the query, determine how

it is accessed: scan, index? Sorts or intermediary results

Check if the plan includes a sorting (because of DISTINCT, ORDER BY, GROUP BY) and see if it can be eliminated

Materialization of temporary results should be avoided

Order of operations May want to force an ordering if we otice the order

does not favor the early reduction in the number of tuples

Algorithms used in the operators

Investigating High Level Consumers

Answer question 1: “Are critical queries being served in the most efficient manner?”

1. Identify the critical queries

2. Analyze their access plans

3. Profile their execution

Profiling a Query’s Execution

A query was found critical but its plan looks okay. What’s going on? Put it to run.

Profiling it would determine the quantity of the resources used by a query and assessing how efficient this use was Profile of a query execution consists of detailed

information about the duration and resource consumption

Resources DBMS subsystems: cache, disk, lock, log OS raw resources: CPU

Query profiling indicators

Duration information involves 3 indicators: Elapsed time for the query: time it took to process it as

perceived by the user CPU time: time the CPU was actually used to process the

query Wait time: time the query was not processing and waiting

for a resource to become available Resource consumption information includes:

(I/O) Physical and logical reads/writes (Locking) Maximum nb locks held, nb lock escalations,nb

deadlocks/timeouts, total time spent waiting for locks (SQL activity) Nb sorts and temporary area usage: gives an

idea of the time spent in expensive overhead activity

Two common scenarios1. Elapsed query time close CPU time, wait time negligible.

Consumption fair: reasonable nb pages being accesses, most of them are logical accesses; nb locks low or inexistent and no deadlocks or lock escalations; if there were sorts, they didn’t augment the nb of logical/physical reads/writes

Access plan is executed in the best way possible; if the rest of the chain is balanced, this is the best performance the system can deliver for this query

2. Noticeable discrepancy between elapsed and CPU time, wait time fills the gap. So there must be a problem in resource consumption: contention pb (probably waiting for locks) or poorly performing resource (ex. If area allocated for sorting is small, then additional I/O).

To distinguish between contention and physical resource consumption problem, run the query in isolation

Performance Monitors to Profiling Queries

Access or compute performance indicators’ values at any time Many, many flavors

Scope of the indicators: Generic (all indicators from OS and DBMS) or specific (correlated indicators of a given subsystem or a given query) Choose the generic one when interested in how several distinct

indicators behave together Frequency of data gathering:

snapshot perf indicators: allow to freeze a situation at a specific moment in time

Continuous perf. Indicator: snaphotting at regular intervals Alarm modes: allows us to enter thresholds beyond which the

system is acting abnormally Presentation of data: textual or graphical

Textual good for seeing the precise values of indicators Graphical chosen when wants to see the trends

An example Performance Monitor (query level)

• Details of buffer and CPU consumption on a query’s report according to DB2’s Benchmark tool

• Similar tools: MSSQL’s SET STATISTICS switch and Oracle’s SQL Analyze Tool

Statement number: 1

select C_NAME, N_NAME

from DBA.CUSTOMER join DBA.NATION on C_NATIONKEY = N_NATIONKEY

where C_ACCTBAL > 0

Number of rows retrieved is: 136308

Number of rows sent to output is: 0

Elapsed Time is: 76.349 seconds

…

Buffer pool data logical reads = 272618

Buffer pool data physical reads = 131425

Buffer pool data writes = 0

Buffer pool index logical reads = 273173

Buffer pool index physical reads = 552

Buffer pool index writes = 0

Total buffer pool read time (ms) = 71352

Total buffer pool write time (ms) = 0

…

Summary of Results

==================

Elapsed Agent CPU Rows Rows

Statement # Time (s) Time (s) Fetched Printed

1 76.349 6.670 136308 0

Statement number: 1

select C_NAME, N_NAME

from DBA.CUSTOMER join DBA.NATION on C_NATIONKEY = N_NATIONKEY

where C_ACCTBAL > 0

Number of rows retrieved is: 136308

Number of rows sent to output is: 0

Elapsed Time is: 76.349 seconds

…

Buffer pool data logical reads = 272618

Buffer pool data physical reads = 131425

Buffer pool data writes = 0

Buffer pool index logical reads = 273173

Buffer pool index physical reads = 552

Buffer pool index writes = 0

Total buffer pool read time (ms) = 71352

Total buffer pool write time (ms) = 0

…

Summary of Results

==================

Elapsed Agent CPU Rows Rows

Statement # Time (s) Time (s) Fetched Printed

1 76.349 6.670 136308 0

An example Performance Monitor (system level)

• An IO indicator’s consumption evolution (qualitative and quantitative) according to DB2’s System Monitor

• Similar tools: Window’s Performance Monitor and Oracle’s Performance Manager

Investigating Intermediate and Primary Resources: routine monitoring

Answer Q3

Problems at

OS level?Answer Q2

Tune low-levelresources

yesno

Problematic

subsystems?Tune

subsystemsInvestigateupper level

yes no

Investigating Primary Resources

• Answer question 3:“Are there enough primary resources available for a DBMS to consume?”

• Primary resources: CPU, disk/controllers, memory, and network

• Analyze specific OS-level indicators to discover bottlenecks.

• A system-level performance monitor is the right tool here

CPU Consumption Indicators at the OS Level

100%

CPU% of

utilization

70%

time

Sustained utilizationover 70% should trigger the alert.

System utilization shouldn’t be more

than 40%.DBMS (in a non-

dedicated machine)should be getting a decent time share.

total usage

system usage

Disk Performance Indicators at the OS Level

Wait queue

Average Queue Size

New requestsDisk Transfers

/second

Should beclose to zero

Wait timesshould also be close to

zeroIdle disk with pending requests?Check controller

contention.Also, transfers

should be balanced amongdisks/controllers

What to do in case of problem? If there are long waiting queues and long service times, check the number of bytes transferred to confirm the disk is busy. Idle disks with pending requests imply controller problems

Also compare the wait times from each disk, if we have a group of disks serving the same purpose

Only reorganize data if we are sure the I/o problems are not dur to high-level consumer inefficiency

Memory Consumption Indicators at the OS Level

pagefile

real memory

virtualmemory

Page faults/timeshould be close

to zero. If paginghappens, at least

not DB cache pages.

% of pagefile inuse (it’s used a fixedfile/partition) will tell

you how much memory is “lacking”.

What to do in case of problems? If the DB buffer size was properly configured,

paging should come from non-DBMS sources.

Checking memory at the OS level should be done whenever we change the memory allocation parameters of the DBMS or there are significant changes in the nb of users

Investigating Intermediate and Primary Resources: routine monitoring

Answer Q3

Problems at

OS level?Answer Q2

Tune low-levelresources

yesno

Problematic

subsystems?Tune

subsystemsInvestigateupper level

yes no

Investigating Intermediate Resources/Consumers

• Answer question 2:“Are subsystems making optimal use of resources?”

• Main subsystems: Cache Manager, Disk subsystem, Lock subsystem, and Log/Recovery subsystem

• Extract and analyze relevant perf. indicators

• A Performance Monitor is usually useful, but sometimes specific tools apply

Cache Manager Performance Indicators

Tablescan

readpage()

Free Page slots

Page reads/writes

Pickvictim

strategy Data Pages

CacheManager

If page is not in the cache, readpage

(logical) generate an actual IO (physical).

Ratio of readpages that did not generate

physical IO (cache hit ration) should be 90%

or more

Pages are regularly saved to disk to make

free space.# of free slots should

always be > 0

What to do in case of problem? If indicators have bad values, the buffer manager is not saving as many disk accesses as it should One or a small nb of queries may be responsible May have slow disk subsystem May have insufficient memory

Tuning parameters accordingly

Disk Manager Performance Indicators

rows

page

extent

file

Storage Hierarchy (simplified)

disk

Row displacement: should kept under 5% of rows

Free space fragmentation: pages with few space should

not be in the free list

Data fragmentation: ideally files that store DB objects

(table, index) should be in one or few (<5) contiguous extents

File position: should balance workload evenly among all

disks

What to do in case of problem? Bad indicator values usually mean inadequately chosen storage parameters To choose good storage parameters, should be aware of rate

at which data grows and with what mix of inserts/updates/deletes that growth rate happens

Also, identifiy occasional but predicatble fluctuations in the DB activity

Then tune disk subsystem

Lock Manager Performance Indicators

Lock request

Object Lock Type TXN ID

LockList

Lockspending list

Deadlocks and timeouts should be virtually

inexistent or extremely low (no more then 1% of the

transactions)

Lock wait time for a transaction should be a

small fraction of the whole transaction time.

Number of locks on waitshould be a small fraction of the number of locks on the lock list, under 20%

What to do in case of problem? The lock subsystem is often a reflection of how well the transactions were designed and seldom a source of a problem itself

Can move up the consumption chain and find expensive queries, including data locking ones.

Logging Subsystem Performance Indicatores Log is used by every transaction that alters, inserts

or deletes data Well tuned logging subsyst. Can write log records at

a pace that doesn’t slow active transactions Indicatores to measure:

Number of log waits to confirm that the disks storing the log files are keeping pace with the transaction workload – should be zero

Nb of log expansions or log archives due to lack of space – if bigger than zero indicates a configuration problem

Log cache-hit ratio to check whether the size of the log buffer is adequate

Conclusion

Monitoring a DBMS’s performance can be done in a systematic way The consumption chain helps distinguishing

problems’ causes from their symptoms Existing tools help extracting relevant

performance indicators The three questions guide the whole monitoring

process