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From Ilya Maykov <ivmay...@gmail.com>
Subject Re: Overwhelming a cluster with writes?
Date Tue, 06 Apr 2010 04:56:08 GMT
I just tried the same test with ConsistencyLevel.ALL, and the problem
went away - the writes are somewhat slower but the cluster never gets
into a bad state. So, I wonder if this is a bug in Cassandra's
handling of async / "non-ConsistencyLevel.ALL" writes ...

-- Ilya

On Mon, Apr 5, 2010 at 9:31 PM, Ilya Maykov <ivmaykov@gmail.com> wrote:
> Hi all,
>
> I've just started experimenting with Cassandra to get a feel for the
> system. I've set up a test cluster and to get a ballpark idea of its
> performance I wrote a simple tool to load some toy data into the
> system. Surprisingly, I am able to "overwhelm" my 4-node cluster with
> writes from a single client. I'm trying to figure out if this is a
> problem with my setup, if I'm hitting bugs in the Cassandra codebase,
> or if this is intended behavior. Sorry this email is kind of long,
> here is the TLDR version:
>
> While writing to Cassandra from a single node, I am able to get the
> cluster into a bad state, where nodes are randomly disconnecting from
> each other, write performance plummets, and sometimes nodes even
> crash. Further, the nodes do not recover as long as the writes
> continue (even at a much lower rate), and sometimes do not recover at
> all unless I restart them. I can get this to happen simply by throwing
> data at the cluster fast enough, and I'm wondering if this is a known
> issue or if I need to tweak my setup.
>
> Now, the details.
>
> First, a little bit about the setup:
>
> 4-node cluster of identical machines, running cassandra-0.6.0-rc1 with
> the fixes for CASSANDRA-933, CASSANDRA-934, and CASSANDRA-936 patched
> in. Node specs:
> 8-core Intel Xeon E5405@2.00GHz
> 8GB RAM
> 1Gbit ethernet
> Red Hat Linux 2.6.18
> JVM 1.6.0_19 64-bit
> 1TB spinning disk houses both commitlog and data directories (which I
> know is not ideal).
> The client machine is on the same local network and has very similar specs.
>
> The cassandra nodes are started with the following JVM options:
>
> ./cassandra JVM_OPTS="-Xms6144m -Xmx6144m -XX:+UseConcMarkSweepGC -d64
> -XX:NewSize=1024m -XX:MaxNewSize=1024m -XX:+DisableExplicitGC"
>
> I'm using default settings for all of the tunable stuff at the bottom
> of storage-conf.xml. I also selected my initial tokens to evenly
> partition the key space when the cluster was bootstrapped. I am using
> the RandomPartitioner.
>
> Now, about the test. Basically I am trying to get an idea of just how
> fast I can make this thing go. I am writing ~250M data records into
> the cluster, replicated at 3x, using Ran Tavory's Hector client
> (Java), writing with ConsistencyLevel.ZERO and
> FailoverPolicy.FAIL_FAST. The client is using 32 threads with 8
> threads talking to each of the 4 nodes in the cluster. Records are
> identified by a numeric id, and I'm writing them in batches of up to
> 10k records per row, with each record in its own column. The row key
> identifies the bucket into which records fall. So, records with ids 0
> - 9999 are written to row "0", 10000 - 19999 are written to row
> "10000", etc. Each record is a JSON object with ~10-20 fields.
>
> Records: {  // Column Family
>   0 : {  // row key for the start of the bucket. Buckets span a range
> of up to 10000 records
>     1 : "{ /* some JSON */ }",  // Column for record with id=1
>     3 : "{ /* some more JSON */ }",  // Column for record with id=3
>    ...
>    9999 : "{ /* ... */ }"
>   },
>  10000 : {  // row key for the start of the next bucket
>    10001 : ...
>    10004 :
> }
>
> I am reading the data out of a local, sorted file on the client, so I
> only write a row to Cassandra once all records for that row have been
> read, and each row is written to exactly once. I'm using a
> producer-consumer queue to pump data from the input reader thread to
> the output writer threads. I found that I have to throttle the reader
> thread heavily in order to get good behavior. So, if I make the reader
> sleep for 7 seconds every 1M records, everything is fine - the data
> loads in about an hour, half of which is spent by the reader thread
> sleeping. In between the sleeps, I see ~40-50 MB/s throughput on the
> client's network interface while the reader is not sleeping, and it
> takes ~7-8 seconds to write each batch of 1M records.
>
> Now, if I remove the 7 second sleeps on the client side, things get
> bad after the first ~8M records are written to the client. Write
> throughput drops to <5 MB/s. I start seeing messages about nodes
> disconnecting and reconnecting in Cassandra's system.log, as well as
> lots of GC messages:
>
> ...
>  INFO [Timer-1] 2010-04-06 04:03:27,178 Gossiper.java (line 179)
> InetAddress /10.15.38.88 is now dead.
>  INFO [GC inspection] 2010-04-06 04:03:30,259 GCInspector.java (line
> 110) GC for ConcurrentMarkSweep: 2989 ms, 55326320 reclaimed leaving
> 1035998648 used; max is 1211170816
>  INFO [GC inspection] 2010-04-06 04:03:41,838 GCInspector.java (line
> 110) GC for ConcurrentMarkSweep: 3004 ms, 24377240 reclaimed leaving
> 1066120952 used; max is 1211170816
>  INFO [Timer-1] 2010-04-06 04:03:44,136 Gossiper.java (line 179)
> InetAddress /10.15.38.55 is now dead.
>  INFO [GMFD:1] 2010-04-06 04:03:44,138 Gossiper.java (line 568)
> InetAddress /10.15.38.55 is now UP
>  INFO [GC inspection] 2010-04-06 04:03:52,957 GCInspector.java (line
> 110) GC for ConcurrentMarkSweep: 2319 ms, 4504888 reclaimed leaving
> 1086023832 used; max is 1211170816
>  INFO [Timer-1] 2010-04-06 04:04:19,508 Gossiper.java (line 179)
> InetAddress /10.15.38.242 is now dead.
>  INFO [Timer-1] 2010-04-06 04:05:03,039 Gossiper.java (line 179)
> InetAddress /10.15.38.55 is now dead.
>  INFO [GMFD:1] 2010-04-06 04:05:03,041 Gossiper.java (line 568)
> InetAddress /10.15.38.55 is now UP
>  INFO [GC inspection] 2010-04-06 04:05:08,539 GCInspector.java (line
> 110) GC for ConcurrentMarkSweep: 2375 ms, 39534920 reclaimed leaving
> 1051620856 used; max is 1211170816
> ...
>
> Finally followed by this and some/all nodes going down:
>
> ERROR [COMPACTION-POOL:1] 2010-04-06 04:05:05,475
> DebuggableThreadPoolExecutor.java (line 94) Error in executor
> futuretask
> java.util.concurrent.ExecutionException: java.lang.OutOfMemoryError:
> Java heap space
>        at java.util.concurrent.FutureTask$Sync.innerGet(Unknown Source)
>        at java.util.concurrent.FutureTask.get(Unknown Source)
>        at org.apache.cassandra.concurrent.DebuggableThreadPoolExecutor.afterExecute(DebuggableThreadPoolExecutor.java:86)
>        at org.apache.cassandra.db.CompactionManager$CompactionExecutor.afterExecute(CompactionManager.java:582)
>        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(Unknown Source)
>        at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
>        at java.lang.Thread.run(Unknown Source)
> Caused by: java.lang.OutOfMemoryError: Java heap space
>        at java.util.Arrays.copyOf(Unknown Source)
>        at java.io.ByteArrayOutputStream.write(Unknown Source)
>        at java.io.DataOutputStream.write(Unknown Source)
>        at org.apache.cassandra.io.IteratingRow.echoData(IteratingRow.java:69)
>        at org.apache.cassandra.io.CompactionIterator.getReduced(CompactionIterator.java:138)
>        at org.apache.cassandra.io.CompactionIterator.getReduced(CompactionIterator.java:1)
>        at org.apache.cassandra.utils.ReducingIterator.computeNext(ReducingIterator.java:73)
>        at com.google.common.collect.AbstractIterator.tryToComputeNext(AbstractIterator.java:135)
>        at com.google.common.collect.AbstractIterator.hasNext(AbstractIterator.java:130)
>        at org.apache.commons.collections.iterators.FilterIterator.setNextObject(FilterIterator.java:183)
>        at org.apache.commons.collections.iterators.FilterIterator.hasNext(FilterIterator.java:94)
>        at org.apache.cassandra.db.CompactionManager.doCompaction(CompactionManager.java:299)
>        at org.apache.cassandra.db.CompactionManager$1.call(CompactionManager.java:102)
>        at org.apache.cassandra.db.CompactionManager$1.call(CompactionManager.java:1)
>        at java.util.concurrent.FutureTask$Sync.innerRun(Unknown Source)
>        at java.util.concurrent.FutureTask.run(Unknown Source)
>        ... 3 more
>
> At first I thought that with ConsistencyLevel.ZERO I must be doing
> async writes so Cassandra can't push back on the client threads (by
> blocking them), thus the server is getting overwhelmed. But, I would
> expect it to start dropping data and not crash in that case (after
> all, I did say ZERO so I can't expect any reliability, right?).
> However, I see similar slowdown / node dropout behavior when I set the
> consistency level to ONE. Does Cassandra push back on writers under
> heavy load? Is there some magic setting I need to tune to have it not
> fall over? Do I just need a bigger cluster? Thanks in advance,
>
> -- Ilya
>
> P.S. I realize that it's still handling a LOT of data with just 4
> nodes, and in practice nobody would run a system that gets 125k writes
> per second on top of a 4 node cluster. I was just surprised that I
> could make Cassandra fall over at all using a single client that's
> pumping data at 40-50 MB/s.
>

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