We start by looking at distributed database features that impact latency. Then we take a deeper look at the HBase read and write paths with a focus on request latency. We examine the sources of latency and how to minimize them.

1. HBase Low Latency
Nick Dimiduk, Hortonworks (@xefyr)
Nicolas Liochon, Scaled Risk (@nkeywal)
Strata New York, October 17, 2014

2. Agenda
• Latency, what is it, how to measure it
• Write path
• Read path
• Next steps

3. What’s low latency
• Meaning from micro seconds (High Frequency
Trading) to seconds (interactive queries)
• In this talk milliseconds
Latency is about percentiles
• Average != 50% percentile
• There are often order of magnitudes between « average » and « 95
percentile »
• Post 99% = « magical 1% ». Work in progress here.

4. Measure latency
YCSB - Yahoo! Cloud Serving Benchmark
• Useful for comparison between databases
• Set of workload already defined
bin/hbase org.apache.hadoop.hbase.PerformanceEvaluation
• More options related to HBase: autoflush, replicas, …
• Latency measured in micro second
• Easier for internal analysis

5. Why is it important
Durability
Availability
Consistency

6. 3
1
Client
Buffer
0
Server
Buffer
HBase BigTable
OS
Buffer
on GFS1
Traditional
DB Engine
Disk
Durability
2

7. Durability

8. Consistency
Two processes: P1, P2
Counter updated by a P1
v1, then v2, then v3
Eventual consistency allows P1
and P2 to see these events in any
order.
Strong consistency allows only one
order
Google F1 paper, VLDB (2013)
We store financial data and have hard requirements
on data integrity and consistency. We also have a lot
of experience with eventual consistency systems at
Google. In all such systems, we find developers spend
a significant fraction of their time building extremely
complex and error-prone mechanisms to cope with
eventual consistency and handle data that may be
out of date. We think this is an unacceptable burden
to place on developers and that consistency problems
should be solved at the database level.

9. Consistency
Big Table design: allows consistency by partitioning the data: each
machine serves a subset of the data.

10. Availability
• Contract is: « a client outside the cluster will sees HBase as available if
there are partitions or failure within the HBase cluster »
• There is a lot more to say, but it’s outside the scope of this talk
(unfortunately)

11. Availability
A partition or a machine failure
appear to the client as a latency spike

12. Trade off
• Maximizing the benefits while minimizing
the cost
• Implementation details count
• Configuration counts

13. Write path
• Two parts
• Single put (WAL)
• The client just sends the put
• Multiple puts from the client (new behavior since 0.96)
• The client is much smarter
• Four stages to look at for latency
• Start (establish tcp connections, etc.)
• Steady: when expected conditions are met
• Machine failure: expected as well
• Overloaded system

14. Single put: communication & scheduling
• Client: TCP connection to the server
• Shared: multitheads on the same client are using the same TCP connection
• Pooling is possible and does improve the performances in some circonstances
• hbase.client.ipc.pool.size
• Server: multiple calls from multiple threads on multiple machines
• Can become thousand of simultaneous queries
• Scheduling is required

15. Single put: real work
• The server must
• Write into the WAL queue
• Sync the WAL queue (HDFS flush)
• Write into the memstore
• WALs queue is shared between all the regions/handlers
• Sync is avoided if another handlers did the work
• You may flush more than expected

16. Simple put: A small run
Percentile Time in ms
Mean 1.21
50% 0.95
95% 1.50
99% 2.12

17. Latency sources
• Candidate one: network
• 0.5ms within a datacenter
• Much less between nodes in the same rack
Percentile Time in ms
Mean 0.13
50% 0.12
95% 0.15
99% 0.47

18. Latency sources
• Candidate two: HDFS Flush
Percentile Time in ms
Mean 0.33
50% 0.26
95% 0.59
99% 1.24
• We can still do better: HADOOP-7714 & sons.

19. Latency sources
• Millisecond world: everything can go wrong
• JVM
• Network
• OS Scheduler
• File System
• All this goes into the post 99% percentile
• Requires monitoring
• Usually using the latest version shelps.

20. Latency sources
• Split (and presplits)
• Autosharding is great!
• Puts have to wait
• Impacts: seconds
• Balance
• Regions move
• Triggers a retry for the client
• hbase.client.pause = 100ms since HBase 0.96
• Garbage Collection
• Impacts: 10’s of ms, even with a good config
• Covered with the read path of this talk

21. From steady to loaded and overloaded
• Number of concurrent tasks is a factor of
• Number of cores
• Number of disks
• Number of remote machines used
• Difficult to estimate
• Queues are doomed to happen
• hbase.regionserver.handler.count
• So for low latency
• Replable scheduler since HBase 0.98 (HBASE-8884). Requires specific code.
• RPC Priorities: since 0.98 (HBASE-11048)

22. From loaded to overloaded
• MemStore takes too much room: flush, then blocksquite quickly
• hbase.regionserver.global.memstore.size.lower.limit
• hbase.regionserver.global.memstore.size
• hbase.hregion.memstore.block.multiplier
• Too many Hfiles: block until compactions keep up
• hbase.hstore.blockingStoreFiles
• Too manyWALs files: Flush and block
• hbase.regionserver.maxlogs

23. Machine failure
• Failure
• Dectect
• Reallocate
• Replay WAL
• ReplayingWAL is NOT required for puts
• hbase.master.distributed.log.replay
• (default true in 1.0)
• Failure = Dectect + Reallocate + Retry
• That’s in the range of ~1s for simple failures
• Silent failures leads puts you in the 10s range if the hardware does not help
• zookeeper.session.timeout

24. Single puts
• Millisecond range
• Spikes do happen in steady mode
• 100ms
• Causes: GC, load, splits

25. Streaming puts
Htable#setAutoFlushTo(false)
Htable#put
Htable#flushCommit
• As simple puts, but
• Puts are grouped and send in background
• Load is taken into account
• Does not block

26. Multiple puts
hbase.client.max.total.tasks (default 100)
hbase.client.max.perserver.tasks (default 5)
hbase.client.max.perregion.tasks (default 1)
• Decouple the client from a latency spike of a region server
• Increase the throughput by 50% compared to old multiput
• Makes split and GC more transparent

27. Conclusion on write path
• Single puts can be very fast
• It’s not a « hard real time » system: there are spikes
• Most latency spikes can be hidden when streaming puts
• Failure are NOT that difficult for the write path
• No WAL to replay

28. And now for the read path

29. Read path
• Get/short scan are assumed for low-latency operations
• Again, two APIs
• Single get: HTable#get(Get)
• Multi-get: HTable#get(List<Get>)
• Four stages, same as write path
• Start (tcp connection, …)
• Steady: when expected conditions are met
• Machine failure: expected as well
• Overloaded system: you may need to add machines or tune your workload

30. Multi get / Client
Group Gets by
RegionServer
Execute them
one by one

31. Multi get / Server

32. Multi get / Server

33. AcceSstso rlaagtee hniecrya rmchya: gan diifdfeeresnt view
Dean/2009
Memory is 100000x
faster than disk!
Disk seek = 10ms

34. Known unknowns
• For each candidate HFile
• Exclude by file metadata
• Timestamp
• Rowkey range
• Exclude by bloom filter
StoreFileScanner#
shouldUseScanner()

35. Unknown knowns
• Merge sort results polled from Stores
• Seek each scanner to a reference KeyValue
• Retrieve candidate data from disk
• Multiple HFiles => mulitple seeks
• hbase.storescanner.parallel.seek.enable=true
• Short Circuit Reads
• dfs.client.read.shortcircuit=true
• Block locality
• Happy clusters compact!
HFileBlock#
readBlockData()

36. BlockCache
• Reuse previously read data
• Maximize cache hit rate
• Larger cache
• Temporal access locality
• Physical access locality
BlockCache#getBlock()

37. BlockCache Showdown
• LruBlockCache
• Default, onheap
• Quite good most of the time
• Evictions impact GC
• BucketCache
• Offheap alternative
• Serialization overhead
• Large memory configurations
http://www.n10k.com/blog/block
cache-showdown/
L2 off-heap BucketCache
makes a strong showing

38. Latency enemies: Garbage Collection
• Use heap. Not too much. With CMS.
• Max heap
• 30GB (compressed pointers)
• 8-16GB if you care about 9’s
• Healthy cluster load
• regular, reliable collections
• 25-100ms pause on regular interval
• Overloaded RegionServer suffers GC overmuch

39. Off-heap to the rescue?
• BucketCache (0.96, HBASE-7404)
• Network interfaces (HBASE-9535)
• MemStore et al (HBASE-10191)

40. Latency enemies: Compactions
• Fewer HFiles => fewer seeks
• Evict data blocks!
• Evict Index blocks!!
• hfile.block.index.cacheonwrite
• Evict bloom blocks!!!
• hfile.block.bloom.cacheonwrite
• OS buffer cache to the rescue
• Compactected data is still fresh
• Better than going all the way back to disk

41. Failure
• Detect + Reassign + Replay
• Strong consistency requires replay
• Locality drops to 0
• Cache starts from scratch

42. Hedging our bets
• HDFS Hedged reads (2.4, HDFS-5776)
• Reads on secondary DataNodes
• Strongly consistent
• Works at the HDFS level
• Timeline consistency (HBASE-10070)
• Reads on « Replica Region »
• Not strongly consistent

43. Read latency in summary
• Steady mode
• Cache hit: < 1 ms
• Cache miss: + 10 ms per seek
• Writing while reading => cache churn
• GC: 25-100ms pause on regular interval
Network request + (1 - P(cache hit)) * (10 ms * seeks)
• Same long tail issues as write
• Overloaded: same scheduling issues as write
• Partial failures hurt a lot

44. HBase ranges for 99% latency
Put
Streamed
Multiput Get Timeline get
Steady milliseconds milliseconds milliseconds milliseconds
Failure seconds seconds seconds milliseconds
GC
10’s of
milliseconds milliseconds
10’s of
milliseconds milliseconds

45. What’s next
• Less GC
• Use less objects
• Offheap
✓Compressed BlockCache (HBASE-11331)
• Prefered location (HBASE-4755)
• The « magical 1% »
• Most tools stops at the 99% latency
• What happens after is much more complex

46. 35.0x
30.0x
25.0x
20.0x
15.0x
10.0x
5.0x
0.0x
Performance with Compressed BlockCache
Total RAM: 24G LruBlockCache Size: 12G
Data Size: 45G Compressed Size: 11G
Compression: SNAPPY
throughput (ops/sec) latency (ms, p95) latency (ms, p99) cpu load
Times improvement
Metric
Enabled
Disabled

47. Thanks!
Nick Dimiduk, Hortonworks (@xefyr)
Nicolas Liochon, Scaled Risk (@nkeywal)
Strata New York, October 17, 2014