Jvm heap

Java™ Garbage CollectionStatistical Analysis 101.

Juarez JuniorSystem Architect – Unisys Global Outsourcing

Agenda.Java Heap Management

Simple Garbage Collection Statistics

• Generating the Data

• Isolating the Data

• Graphing the Data

• Interpreting the Data

• Improving Performance

Advanced Garbage Collection Statistics

• Generate, Isolate, Graph, Interpret, Improve

Java Command Line Arguments Recap

Java Heap.Young Generation

• Eden

- Where new objects are created

• Survivor spaces

- Where garbage collector places objects that are still in use

Old Generation

• Where tenured objects are placed

References

• http://java.sun.com/docs/hotspot/gc/ (1.3.1)

• http://java.sun.com/docs/hotspot/gc1.4.2/

Minor Garbage Collection.Occurs when no room in eden for new object

Marks all reachable objects in eden and “from” survivor space

Copies those objects to “to” survivor space

• Updates references accordingly

• If “to” survivor space fills, overflows to old generation

Resets allocation pointer to start of eden

Old Generation EdenSpaces

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E F

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To From

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From To

Major Garbage Collection.Occurs when insufficient room in old generation to hold to-be-tenured objects during a minor collection

• Pessimistic – need sufficient space to hold all object in young generation

• Pre-calculated – Minor collection reachable object algorithm run to determine exactly now many objects will be tenured

Mark and Compact

• Reachable objects are marked

• Marked objects are moved to one end of the old generation

Old Generation EdenSpaces

To From

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J K

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Generating Simple GC Data.-verbose:gc command line option

java ... –verbose:gc ... my.java.app ...

Results:

Minorcollections

Major and minor

collection

...[GC 1860K->1388K(1984K), 0.0005059 secs][GC 1900K->1446K(1984K), 0.0006679 secs][GC 1958K->1468K(2112K), 0.0006251 secs][Full GC 1468K-> 195K(2112K), 0.0131045 secs]...

Time it took

Total heap size

Heap in use after collection

Heap in use before collection

Isolating the Simple GC Data....[GC 1860K->1388K(1984K), 0.0005059 secs][GC 1900K->1446K(1984K), 0.0006679 secs][GC 1958K->1468K(2112K), 0.0006251 secs][Full GC 1468K-> 195K(2112K), 0.0131045 secs]...

Analyzer

... 1860,1388,0.0005059 1900,1446,0.0006679 1958,1468,0.0006251 1468,195,0.0131045 ...

import java.util.regex.Matcher;import java.util.regex.Pattern;...Pattern p = Pattern.compile ("\\[(?:Full |)GC (\\d*)K->(\\d*)K\\(\\d*K\\), ([\\d.]*) secs\\]");Matcher m = p.matcher(simple_gc_data_output);while (m.find()) fout.println(m.group(1) + "," + m.group(2) + "," + m.group(3));

Comma-separated value (CSV) file

[Full GC 1468K-> 195K(2112K), 0.0131045 secs]

Interpreting the Regular Expression.The ’\’ char is an escape character, both in Java and in regular expressions.\[(?:Full |)GC (\d*)K->(\d*)K\(\d*K\), ([\d.]*) secs\]

\[(?:Full |)GC - match literal ‘[Full GC ’ or ‘[GC ’

(\d*)K-> - capture all digits before the literal text ‘K->’

(\d*)K - capture all digits before the literal text ‘K’

\(\d*K\) - discard the digits and literal text ‘K’ within parenthesis

, ([\d.]*) secs\] - capture all digits and decimal point after the comma and space, and before the literal text ‘secs]’

Graphing the Simple GC Data.Load the CSV file into a spreadsheet

Sum the third column - total time spent on GC

Create fourth column

• Third column * 10000 (or 100000)

• Gets times within range of columns 1 and 2

Create a graph containing columns1, 2 and 4.

Graphing in Excel.Select first column only

Create an “XY (Scatter)” graph

Wizard step 2, click Series tab and add columns 2 and 4 to the series.

• Click Add

• Name: Type or select row 1 cell

• X Values: leave empty

• Y Values: Select cells in column

Suggest creating new sheet for the chart

Graphing in StarOffice.

Select the first 3 columns and create chart

• Put chart in new sheet

• Select ‘lines’ chart type

• Select ‘normal’ variant

After chart is created

• Turn off labels on the X axis

• Change width of data series lines to 0.02” or higher.

Swap the 3rd and 4th columns (seconds*10000 is now the 3rd column)

Interpreting Graphed GC Data.Typical GC data graph

For many GCs, the heap slowly fills as objects get moved to the old generation

• Blue (or magenta) lines with positive slope

Then a full GC happens

• Drop in blue (or magenta) lines

• Yellow dot in higher position

Interpreting Graphed GC Data.Three Phases

Phase 1

• Working set equilibrium

• Ideal graph

• No full GC

Phase 2

• Full GC with heap not full

- High GC times (yellow dots above blue zig-zag)

• App called System.GC()

Phase 3

• Incremental working set equilibrium

• Implies various sub-phases each of which adds to working set

Phase 1 Phase 2 Phase 3

Improving Performance, Before.Goal: 1000 business transactions in 12 minutes

Heap set to: -Xms1024m –Xmx1024m

18 minute run, 472 seconds (7.8 minutes) in garbage collection

Each GC is freeing very little memory

Improving Performance, After.-Xms1024m –Xmx1024m -XX:NewSize=300m –XX:MaxNewSize=300m

12 minute run, 25 seconds in garbage collection

• 70 collections instead of 8000 (no major/full collections)

• Average collection freed 240MB instead of just 1MB

[GC [DefNew: 17131K->1606K(18432K), 0.0082055 secs] 44904K->29379K(63488K), 0.0083625 secs]

[GC [DefNew: 17990K->17990K(18432K), 0.0000839 secs] [Tenured: 27772K->3759K(45056K), 0.0454394 secs] 45763K->3759K(63488K), 0.0459597 secs]

More GC Data.PrintGCDetails command line option

java ... –XX:+PrintGCDetails ... my.java.app ...

Results:

Minorcollection

Major and minor

collection Time it took

Heap size

Heap in use after collection

Heap in use before collection

Sizes are for young generation, old

generation, and total heap

And Even More GC Data.PrintHeapAtGC command line option

• java ... –XX:+PrintHeapAtGC ... my.java.app ...

Results:{Heap before GC invocations=422:Heap def new generation total 18432K, used 17639K [0x6eb40000, 0x6ff40000, 0x6ff40000) eden space 16384K, 100% used [0x6eb40000, 0x6fb40000, 0x6fb40000) from space 2048K, 61% used [0x6fd40000, 0x6fe79df8, 0x6ff40000) to space 2048K, 0% used [0x6fb40000, 0x6fb40000, 0x6fd40000) tenured generation total 45056K, used 6403K [0x6ff40000, 0x72b40000, 0x72b40000) the space 45056K, 14% used [0x6ff40000, 0x70580f78, 0x70581000, 0x72b40000) compacting perm gen total 8192K, used 1597K [0x72b40000, 0x73340000, 0x76b40000) the space 8192K, 19% used [0x72b40000, 0x72ccf598, 0x72ccf600, 0x73340000) Heap after GC invocations=423:Heap def new generation total 18432K, used 1197K [0x6eb40000, 0x6ff40000, 0x6ff40000) eden space 16384K, 0% used [0x6eb40000, 0x6eb40000, 0x6fb40000) from space 2048K, 58% used [0x6fb40000, 0x6fc6b4e8, 0x6fd40000) to space 2048K, 0% used [0x6fd40000, 0x6fd40000, 0x6ff40000) tenured generation total 45056K, used 6416K [0x6ff40000, 0x72b40000, 0x72b40000) the space 45056K, 14% used [0x6ff40000, 0x70584190, 0x70584200, 0x72b40000) compacting perm gen total 8192K, used 1597K [0x72b40000, 0x73340000, 0x76b40000) the space 8192K, 19% used [0x72b40000, 0x72ccf598, 0x72ccf600, 0x73340000)}

{Heap before GC invocations=422:Heap def new generation total 18432K, used 17639K [0x6eb40000, 0x6ff40000, 0x6ff40000) eden space 16384K, 100% used [0x6eb40000, 0x6fb40000, 0x6fb40000) from space 2048K, 61% used [0x6fd40000, 0x6fe79df8, 0x6ff40000) to space 2048K, 0% used [0x6fb40000, 0x6fb40000, 0x6fd40000) tenured generation total 45056K, used 6403K [0x6ff40000, 0x72b40000, 0x72b40000) the space 45056K, 14% used [0x6ff40000, 0x70580f78, 0x70581000, 0x72b40000) compacting perm gen total 8192K, used 1597K [0x72b40000, 0x73340000, 0x76b40000) the space 8192K, 19% used [0x72b40000, 0x72ccf598, 0x72ccf600, 0x73340000) Heap after GC invocations=423:Heap def new generation total 18432K, used 1197K [0x6eb40000, 0x6ff40000, 0x6ff40000) eden space 16384K, 0% used [0x6eb40000, 0x6eb40000, 0x6fb40000) from space 2048K, 58% used [0x6fb40000, 0x6fc6b4e8, 0x6fd40000) to space 2048K, 0% used [0x6fd40000, 0x6fd40000, 0x6ff40000) tenured generation total 45056K, used 6416K [0x6ff40000, 0x72b40000, 0x72b40000) the space 45056K, 14% used [0x6ff40000, 0x70584190, 0x70584200, 0x72b40000) compacting perm gen total 8192K, used 1597K [0x72b40000, 0x73340000, 0x76b40000) the space 8192K, 19% used [0x72b40000, 0x72ccf598, 0x72ccf600, 0x73340000)}


PrintHeapAtGC Data.{Heap before GC invocations=422:Heap def new generation total 18432K, used 17639K [0x6eb40000, 0x6ff40000, 0x6ff40000) eden space 16384K, 100% used [0x6eb40000, 0x6fb40000, 0x6fb40000) from space 2048K, 61% used [0x6fd40000, 0x6fe79df8, 0x6ff40000) to space 2048K, 0% used [0x6fb40000, 0x6fb40000, 0x6fd40000) tenured generation total 45056K, used 6403K [0x6ff40000, 0x72b40000, 0x72b40000) the space 45056K, 14% used [0x6ff40000, 0x70580f78, 0x70581000, 0x72b40000) compacting perm gen total 8192K, used 1597K [0x72b40000, 0x73340000, 0x76b40000) the space 8192K, 19% used [0x72b40000, 0x72ccf598, 0x72ccf600, 0x73340000) Heap after GC invocations=423:Heap def new generation total 18432K, used 1197K [0x6eb40000, 0x6ff40000, 0x6ff40000) eden space 16384K, 0% used [0x6eb40000, 0x6eb40000, 0x6fb40000) from space 2048K, 58% used [0x6fb40000, 0x6fc6b4e8, 0x6fd40000) to space 2048K, 0% used [0x6fd40000, 0x6fd40000, 0x6ff40000) tenured generation total 45056K, used 6416K [0x6ff40000, 0x72b40000, 0x72b40000) the space 45056K, 14% used [0x6ff40000, 0x70584190, 0x70584200, 0x72b40000) compacting perm gen total 8192K, used 1597K [0x72b40000, 0x73340000, 0x76b40000) the space 8192K, 19% used [0x72b40000, 0x72ccf598, 0x72ccf600, 0x73340000)}

Data from before GC



Data from after GC



Memory addresses

Memory addresses: - start - next - end



Before GC, eden ‘next’ pointer is at end

After GC, eden ‘next’ pointer is at start



PrintHeapAtGC Data.

For each generation

Memory allocated

Memory in use



PrintHeapAtGC Data.

For each generational sub-space

Amount of memory allocated

Percentage of that memory that is in use

Isolating the Data.Extract data

• Need a very complex regular expression

• CSV file, spreadsheet, with many columns

Interesting data

• Memory in use by each generation

- Before and after each collection

• Percent in use of “from space”

- NOTE: “After” data same as next “Before” data

New data worth

graphing

Graphing and Analyzing the Data.“From Space” usage is between 10% and 60%

Adjust from space size using SurvivorRatio option

• -XX:SurvivorRatio=ratio

• Eden-size = survivor-size * ratio

• Eden-size + 2 * survivor-ratio = young-gen-size

Example:

• -XX:NewSize=200M –XX:SurvivorRatio=8

• Eden: 160MB, each Survivor Space: 20MB

Aim for 90-95% usage after collection

• Typically, with large young generations, use larger ratio (e.g. 32)

Garbage Collection Info Options.-verbose:gc

• Heap usage before and after GC

• Time spent during GC

-XX:+ PrintGCDetails

• Generation and heap size before and after GC

• Time spent during GC

-XX:+ PrintHeapAtGC

• Generation sizes before and after GC

• Space sizes and percent in use before and after GC

• Memory locations of the heap, its generations, and their spaces

Java Tuning Options.-Xmssizem, -Xmxsizem

• Minimum and maximum heap size

• Set both sizes to the same value

-XX:NewSize=sizem, -XX:MaxNewSize=sizem

• Minimum and maximum young generation size

• Set both sizes to the same value

• Set to about 1/3 or 1/4 the size of the heap.

-XX:SurvivorRatio=ratio

• Number of times the eden space is larger than the “from” space

• Try for maximum of 90-95% usage

Conclusions.The HotSpot™ JVM provides a variety of options for gathering garbage collection statistics.

Those statistics are easily extracted using

• A simple Java application using

• A regular expression

The extracted data is easily graphed using a spreadsheet application.

Examining the resulting graph

• Tells a lot about what the application is doing

• Enables selection of proper heap sizing to improve performance of the application.

Questions?

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