Keeping Infinispan In Shape: Highly-Precise, Scalable Data Eviction

galder@jboss.org | twitter.com/galderz | zamarreno.com
Thursday, October 7, 2010

Keeping Infinispan In Shape:
Highly-Precise, Scalable
Data Eviction

Galder Zamarreño & Vladimir Blagojevic
Senior Engineer, Red Hat
7th October 2010, JUDCon - Berlin


Who is Galder?

• R&D engineer (Red Hat Inc):
• Infinispan developer
• JBoss Cache developer
• Contributor and committer:
• JBoss AS, Hibernate, JGroups, JBoss Portal,...etc
• Blog: zamarreno.com
• Twitter: @galderz


Agenda

• Eviction in:
• Java Collections Framework
• JBoss Cache
• Infinispan 4.0
• New in Infinispan 4.1:
• LIRS
• Batching Updates


Java Collections Framework

• Key building block of any Java app
• Introduced in Java 1.2
• Extended with concurrent collections in Java 1.5
• Collection element eviction ??


Collections cannot grow forever


If using a collection as cache

• Forces clients to either:
• Remove elements proactively
• Or run a periodic cleanup process
• Which can be a PITA...


Eviction in JBoss Cache days


JBoss Cache eviction issues

• Under heavy load, eviction queues could fill up - bottleneck
• Possibly a side effect of MVCC’s non-blocking reads
• Separating data into regions could alleviate the issue
• But it’s really a hack!


Original Infinispan 4.0 eviction

• Tried a different approach:
• Avoid using queues to hold events
• Taking advantage of tree to map change:
• Attempt to maintain map ordered as per eviction rules
• Could we use ConcurrentSkipListMap ?
• No. O(1) desired for all map operations


Doubly Linked
ConcurrentHashMap

• Based on H.Sundell and P.Tsigas paper:

• Lock-Free Deques and Doubly Linked Lists (2008)

• With each cache access or update update links

• Eviction just a matter of walking the linked list one way


Issues under heavy load

• Algorithm always uses same node (the tail) to append stuff
• With high concurrency, CAS stress lead to loads of retrying
• So much retrying, we’re getting infinite loops
• Before 4.0.0.Final, reverted to a more conservative algorithm


4.0.0.Final eviction


LRU eviction algorithm issues

• Weak access locality
• One-time accessed keys not evicted timely
• In loops, soon to be accessed keys might get evicted first
• With distinct access frequencies, frequently accessed keys
can unfortunately get evicted
• LRU’s working set limited to cache size


Enter the room... LIRS

• Eviction algorithm that can cope with weak access locality
• Based on S.Jiang and X.Zhang’s 2002 paper:
• LIRS: An efficient low inter-reference recency set
replacement policy to improve buffer cache performance
• LIRS based around two concepts: IRR and Recency


IRR and Recency
• Inter-Reference Recency (IRR):
• Number of other unique keys accessed between two
consecutive accesses to same key
• Recency (R)
• Number of other unique keys accessed from last reference
until now


How does LIRS work?

• If key has a high IRR, it’s next IRR is likely to be high again
• Keys with highest IRR are considered for eviction
• Once IRR is out of date, we start relying on Recency
• LIRS = Low Inter-reference Recency Set


How is LIRS implemented?

• Low IRR (LIR) area
• Holds hot keys!
• High IRR (HIR) area
• Holds recently accessed keys
• Keys here might get promoted to LIR area


Cache Hit - LIR area


Hit - HIR area and in LIR Q ...


Hit - HIR area and in LIR Q


Hit - HIR area and not in LIR Q


Cache Miss


LIRS implementation hurdles

• LIRS requires a lot of key shifting around
• It can lead to high contention
• Unless you can implement it in scalable way, it’s useless
• Low contended way to implement a high precision eviction
algorithm? Is it possible?


Batching Eviction Updates
• Original idea: X.Ding, S.Jiang and X.Zhang’s 2009 paper:
• BP-Wrapper: A System Framework Making Any
Replacement Algorithms (Almost) Lock Contention Free
• Keeping cache access per thread in a queue
• If queue reaches a threshold:
• Acquire locks and execute eviction as per algorithm
• Batching updates significantly lowers lock contention


Batching Updates in Infinispan

• Decided against recording access per thread
• 100s threads could be hitting cache; some short lived
• Created BoundedConcurrentHashMap
• Based on Doug Lea's ConcurrentHashMap
• Records accesses in a lock-free queue in each segment
• When threshold passed, acquire lock for segment and evict


Precision and Performance

• Segment level eviction does not affect overall precision
• Community member run some performance tests:
• After swapping their own LRU cache with BoundedCHM:
• Berlin SPARQL Benchmark performance increased
55-60% for both cold and hot caches


Summary

• In JBoss Cache, eviction can become a bottleneck
• Infinispan 4.0 uses conservative eviction
• Infinispan 4.1 has more precise eviction algorithm (LIRS)
• Batching updates, present in 4.1, significantly lowers lock
contention
• Result = Highly-concurrent, highly-precise implicit eviction


Questions?

infinispan.org
blog.infinispan.org
twitter.com/infinispan
#infinispan
#judcon


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