Ece2013 Java Advanced Memorymanagement

Java -‐ Advanced Memory Management

Chris6an Campo -‐ EclipseCon Europe 2013

Memory API

Copyright © 2013 compeople AG, Made available under the Eclipse Public License v 1.0

•  Overview •  Challenges •  Cache Problems •  API

Agenda


Overview

Private filesystem

Hierarchical datastructure • root, folders, files, attributes, binary content

Simultaneous read/write from multiple threads • application UI, synchronizer, filesystem driver etc.

Stored in sql database

Schema is dynamic (columns added at runtime)

No OR-Mapper


Data model

<root>

"docs" "images"

"a.txt"

/docs

/docs/a.txt

"/"

/images

Attributes: -  lastmodified -  length -  owner -  customernumber


Components

Store API

Files / Directories

Nodes

SQL Access

Application Virtual File System Synchronizer(Daemon)


•  Repe66ve queries (aHributes) › Windows assumes any FileSystem opera6on as cheap

•  Applica6on developer don't think about internals ›  they assume that any API call is cheap and fast

•  Data structure is dynamic •  Growing size of data

Challenges


Developer stereotypes

DB Developer • ACID • transactions • logic should be a stored procedure

• db has buffer (=cache)

Framework Developer • clean and small API • refactor often • DBs are too slow à cache

Application Developer • never refactor :-) • wrap every framework • Framework is too slow à cache

• why cant I access the DB directly ? :-)


Cache -‐ Requirements

Rule #1: Cache improves speed but does not change functionality

Rule #2: Cache must not lead to OOM


Virtual File System -‐ FileCache


often lists children of a

directory

tempting to add a cache

build "MyCache" :-)

Virtual File System -‐ FileCache


VFS -‐ "MyCache" V1.0

private WeakHashMap<String, List<GFile> > cache = new WeakHashMap<String, List<GFile> >();

private Store store; public synchronized List<GFile> list( final GFile gFile ) { List<GFile> list = cache.get(gFile.getPath()); if ( list == null) { list = store.list(goyaFile ); cache.put(gFile.getPath(), list ); } return list; }


VFS -‐ "MyCache" V1.0 -‐ junit test

// create + write content for (int x=0; x<100; x++) { GFile file = store.create(new GFile("/docs/test" +x+ ".txt")); } // check for (int x=0; x<100; x++) { List<GFile> list = list(new GFile("/docs")); assertTrue(list.contains(new GFile("/docs/test" +x+ ".txt"))); } // list from "MyCache" public synchronized List<GFile> list( final GFile gFile ) {}

OK + FASTER with Cache


VFS -‐ "MyCache" V1.0 -‐ junit test

// create + write content + check for (int x=0; x<100; x++) { GFile file = store.create(new GFile("/docs/test“ +x+ ".txt")); List<GFile> list = list(new GFile("/docs")); assertTrue(list.contains(new GFile("/docs/test“ +x+ ".txt"))); } // list from "MyCache" public synchronized List<GFile>

list( final Store store, final GFile gFile ) {}

FAILS with Cache -> expiration time for cache entries


private long EXPIRATION_TIME = 200; private WeakHashMap<String, MyCacheEntry> cache =

new WeakHashMap<String, MyCacheEntry>(); public synchronized List<GFile> list( final LocalStore store, final GFile gFile ) { final MyCacheEntry myCacheEntry = cache.get(gFile.getPath()); if ( myCacheEntry == null || System.currentTimeMillis() - EXPIRATION_TIME > myCacheEntry.currentTimeMillis ) { final List<GoyaFile> listGoyaFiles = store.list( goyaFile ); cache.put(gFile.getPath(), new MyCacheEntry( listGoyaFiles ) ); return listGoyaFiles; } else { return myCacheEntry.listGoyaFiles; }

VFS -‐ MyCache V2.0

class MyCacheEntry { public List<GFile> listGFiles; public long currentTime; MyCacheEntry( final List<GFile> listGFiles ) { this.listGFiles = listGFiles; this.currentTime = System.currentTimeMillis(); } }


•  It was hard to find a good value for EXPIRATION_TIME › 200 ms effec6ve but it was easy to get wrong HITS

› 10 ms beFer (not always right) but not many HITS •  implement a maximum number of entries

•  find means to "know" when CacheEntries are invalid •  sta6s6cs ?

Virtual FS -‐ MyCache V2.0 -‐ more problems ...


Virtual FS -‐ LoadingCache (Guava)

private final LoadingCache<String, List<GFile>> cache = CacheBuilder.newBuilder()

.maximumSize( 10 ) .softValues()

.expireAfterWrite( 200, TimeUnit.MILLISECONDS) .build( new CacheLoader<String, List<GFile>>() { public List<GFile> load( final String path ) {

return store.list( new GFile( path ) ); }

} );

Rule #3: Never write your own Cache implementation


Virtual FS -‐ invalida6ng CacheEntries

public GFile create(GFile file) { GFile g = store.create(file); cache.flush(); return g; }

public GFile create(GFile file) { GFile g = store.create(file); cache.remove(getParent(file).getPath()); return g; }

Rule #4: Have a strategy for removing cache entries (expiration timing alone does not work)


AHribute Cache


•  heavy load on the db •  a lot of read opera6ons on aHributes •  not many writes •  à cache aHributes

AHribute Cache


AHribute Cache

private final Map<String, String> attributesCache; public String getAttribute( final String name ) { String value = attributesCache.get( name ); if (value!=null) { return value; } ... // read it from DB } public void setAttribute( final String name, String value) { if ( !Objects.equal( value, attributesCache.get( name ) ) ) { attributesCache.put( name, value ); ... // update attribute in the database } }


AHribute Cache -‐ Junit test

// previous value of length = "1057" db.transaction(new DbRunnable() { protected void run() { file.setAttribute("length", "0"); long newLength = store.put(file, inputStream); file.setAttribute("length", new String(newLength)) } }); ...

•  IOException reading inputStream •  transaction rolls back •  attribute length = "0" in Cache and has "1057" in DB •  setAttribute("length","0") after Transaction, does not reach DB

public void setAttribute(final String name,String value) { if(!Objects.equal(value, attributeCache.get(name))){ attributesCache.put( name, value ); ... // update attribute in the database }

Rule #5: Cache should be aware of transaction rollbacks

file.setAttribute("length", "0");


AHribute Cache -‐ Transac6ons

private final Map<String, Optional<String>> attributesCache; private void init() { getTransaction().register(this); } public String getAttribute( final String key ) { ... } public void setAttribute( final String name, String value) {} public void rollback() { // called when transaction rolls back attributesCache.flush(); }


AHribute Cache -‐ Transac6ons

db.transaction(new DbRunnable() { try { store.put(file, inputStream); file.setAttribute("loaded","true"); } catch (IOException ioe) { file.setAttribute("loaded","false"); throw ioe; } });

•  IOException -> transaction rollback •  loaded = "false" NOT in Cache, NOT in DB

•  Cache changes behaviour once it does Transaction rollback. •  Rule #6 Dont return with Exception if data should not rollback


AHribute Cache -‐ Isola6on

Transaction 1 Component A

Transaction 2 Component B

???

x = ?

GFile


AHributeCache -‐ Isola6on -‐ Solu6on 1

public void setAttribute( final String name, String value ) { ... getTransaction().getContext().set( ... + name, value ); ... } public String getAttribute(String name) { ... String value = getTransaction().getContext().get( ... + name ); ... }

•  store cached values global IN the transaction context •  however all attribute values are stored in Transaction


AHributeCache -‐ Isola6on -‐ Solu6on 2 (beHer)

private HashMap<String, String> cache=new HashMap<String,String>(); public void setAttribute( final String name, String value) { ... cache.put(getTransactionId() + name, value); ... } public String getAttribute(String name) { ... String value = cache.get(getTransactionId() + name); ... }

store cached values local depending on transaction-id


ChunkCache -‐ data with dirty state


• VFS writes and reads data in random "segments" ›  i.e. "write offset 110.000 length 200"

•  Stored on disk in 1 MB "Chunks"


01011001010101010101010101010110100011110101

Chunks (1 MB)

Virtual FileSystem

•  "write offset=110.000 length=2000" •  "write..." •  "write..." •  "write..." •  "write..." •  "write..."

ChunkFileStore

01011001010101010101010101010110100011110101

01011001010101010101010101010110100011110101

01011001010101010101010101010110100011110101


•  Cache writes lazy •  Cache maintained in dirty state

•  LoadingCache used •  6meout, maxsize


01011001010101010101010101010110100011110101

Chunks (1 MB)

Virtual FileSystem

•  "write offset=110.000 length=200" •  "write..."

ChunkCache

ChunkFileStore

01011001010101010101010101010110100011110101

01011001010101010101010101010110100011110101

01011001010101010101010101010110100011110101

01011001010101010101010101010110100011110101

01011001010101010101010101010110100011110101



private final LoadingCache<Integer, Chunk> cache = CacheBuilder.newBuilder() .maximumSize(4) .expireAfterAccess(2, TimeUnit.SECONDS) .removalListener(new RemovalListener<Integer, Chunk>() { public void onRemoval(final RemovalNotification<Integer, Chunk> notification ) { if ( notification.getValue().isDirty() ) { // write chunk } } }) .build( new CacheLoader<Integer, Chunk>() { public Chunk load( final Integer key ) throws Exception { // load Chunk for key } } );

•  write to disk on remove •  cache entries only expire on access !! •  no automatic housekeeping


query for resources sa6sfying a condi6on

API -‐ query -‐ list

•  can produce a lot of instances •  returns only upon complete resultset •  returns all result instances •  memory footprint unpredicable (maybe large)

ArrayList<GFile> query("type='file' and lastmodified>'20130901' ");


API -‐ query -‐ lazy list

•  List can be lazy and custom implementation •  that only stores primkeys •  builds instance on demand

•  returns only upon complete resultset •  less memory footprint

List<GFile> query("type='file' and lastmodified>'20130901'");


API -‐ query -‐ iterator

•  Iterator can be lazy and custom implementation •  and builds instance on demand

•  uses DB cursor and reads on demand •  less memory footprint •  result can only be consumed once

Iterator<GFile> query("type='file' and lastmodified>'20130901'");


•  track "framework" objects that the applica6on holds on too •  for sta6s6cs •  for error logging

›  i.e. "more than 10.000 file resources WARN"

•  Implementa6on ›  store all returned object in a WeakHashMap

› GC removes objects when there is no hard reference › WeakHashMap.size() can give you an ESTIMATE

API -‐ track objects "idea"


API -‐ famous "count" implementa6on

WASTE

OK

looks trivial, but its a real world example :-)

public long notYetSyncFiles() { localStore.query("type='file' and changed='true'").size(); }

public long notYetSyncFiles() { localStore.queryCount("type='file' and changed='true'"); }


•  fields that have a limited number of values ›  true / false › bitsets (1,8,128,32) › filetypes 'file' / 'directory' › etc.

•  if you read those a lot from DB or remote services*

› you quickly have 10.000 or more instances of 'true'

API -‐ mapping to String

•  intern() is expensive •  PermGen stores 'intern'

•  small space •  since Java7 in Heap

if (value.equals("true") || value.equals("false") || ... { value = value.intern(value); }


Cache must not change the behaviour

Use existing Cache

implementations

Caches must be aware of database

behaviour (ACID) API must scale

Cache / API -‐ Summary

Ece2013 Java Advanced Memorymanagement

Technology

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