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From Sylvain Lebresne <sylv...@datastax.com>
Subject Re: ReplicateOnWrite issues
Date Tue, 12 Jul 2011 22:02:28 GMT
On Tue, Jul 12, 2011 at 11:42 PM, David Hawthorne <dhawth@gmx.3crowd.com> wrote:
> Well, I was using a large number of clients:  I tried configuring a hector pool of 20-200
to see what affect that had on throughput. There's definitely a point after which there's
no gain, so I dialed it back down.  To clarify a few other things, when I say inserts I mean
increments, as this test only involves counters.  When I say a row is full, I mean I've written
all of the data into it that belongs in it from our application's perspective.  Not all rows
are the same length, and the average number of subcolumns per supercolumn is higher for some
rows than for others.
>

Oh yes, you're using supercolumns. That could help magnify the problem
a little bit, given that to read any subcolumn of a supercolumn, we
have to read the whole supercolumn. Depends how many counters per
supercolumns we're talking about.

> I'm curious, is it necessary for a read to happen for replication when using RF=1?

No, it's not necessary with RF=1 ... but we still do it :(. Ok, it's
basically a bug, we'll
fix that for 0.8.2.

> I am currently running a test against a single node cluster, RF=1, CL.ONE, 4 connections/concurrent
increments, and graphing various data points.  If something interesting happens, I'll send
a graph.
>
> When you say stagger writes, do you mean just lower the overall number of increments/sec
my client is attempting,

Yes, this is what I mean.

>or do you mean I should stagger the load amongst many CFs/rows at once?  Right now I'm
writing heavily into a small set of rows, each going into 3 CFs, before moving on to the next
small set of rows.
>
>
> On Jul 12, 2011, at 12:26 PM, Sylvain Lebresne wrote:
>
>> When you do counter increment at CL.ONE, the write is acknowledged as
>> soon as the first replica getting the the write has pushed the
>> increment into his memtable. However, there is a read happening for
>> the replication to the other replicas (this is necessary to the
>> counter design). What is happening is that our nodes are not able to
>> do those reads fast enough. Or, another way to see it is that you are
>> writing faster than you cluster can handle.
>>
>> I've opened https://issues.apache.org/jira/browse/CASSANDRA-2889 to do
>> something about that, but for now you'll have to stagger your writes.
>> I'm more surprised that tasks on the replicate on write stage would
>> stack up at QUORUM, unless you're using a very large number of
>> clients.
>>
>> --
>> Sylvain
>>
>> On Tue, Jul 12, 2011 at 8:57 AM, David Hawthorne <dhawth@gmx.3crowd.com> wrote:
>>> This is now my fourth attempt to get the message through.  Apologies if you
see multiple copies.
>>>
>>> I've tried to give as much relevant data as I can think of, but please let me
know if you need any other info.  I spent the day getting jmxtrans to talk to statsd with
the cassandra JMX data, so I can provide graphs of just about anything you want.  I can also
re-run the tests to replicate the problem if need be.  As always, thanks for taking a look!
>>>
>>> I have a 4 host test cluster that I'm writing counters into running 0.8.1, and
I'm writing to it with hector with CL.ONE or CL.QUORUM (test was run with each and both gave
the same results).  Partitioner is the RandomPartitioner, and RF=3.  nodetool ring shows
that actual data is well balanced:
>>>
>>> Address         DC          Rack        Status State   Load  
         Owns    Token
>>>                                                    
                         127605887595351923798765477786913079296
>>> 10.0.0.57    datacenter1 rack1       Up     Normal  1.59 GB      
  25.00%  0
>>> 10.0.0.56    datacenter1 rack1       Up     Normal  1.39 GB      
  25.00%  42535295865117307932921825928971026432
>>> 10.0.0.55    datacenter1 rack1       Up     Normal  1.24 GB      
  25.00%  85070591730234615865843651857942052864
>>> 10.0.0.54    datacenter1 rack1       Up     Normal  1.34 GB      
  25.00%  127605887595351923798765477786913079296
>>>
>>> I also tested against a 1-box test cluster with RF=1.
>>>
>>> Both show the same results:  high performance for a while, and then ReplicateOnWrite
backs way up (I've seen it as high as a million), the cassandra process becomes unresponsive,
and the hector client starts throwing exceptions.  High performance in this case equates
to about 5-20k inserts/sec on both the 4-box and 1-box cluster (I'm inserting one long row
at a time, so the performance is bounded by the one box owning the row).  The cassandra process
never seems to recover, even if left for 24 hours.  It's still chewing through those ReplicateOnWrite
pending tasks.
>>>
>>> GC doesn't seem to be a factor.  Logs show GCs completing in < 1s on all
4 boxes in the test cluster, as well as on the 1 standalone server.
>>>
>>> I'm also seeing highly erratic performance from the cluster in general:  inserts/sec
usually start at around 3k, but then maybe creep up towards 6k, then drop back down to below
1k for an extended period of time, then maybe 1% of the time they'll spike up to 8k or even
15k, but that doesn't last more than 30 seconds.
>>>
>>> Here's the tpstats output from all 4 boxes:
>>>
>>> Slowest/most problematic/highest cpu usage one:
>>>
>>> This tpstats output is also representative of the one box cluster about 20 minutes
into my insert run.
>>>
>>> 4 $ ./nodetool -h localhost tpstats
>>> Pool Name                    Active   Pending      Completed
>>> ReadStage                         0         0          
   0
>>> RequestResponseStage              0         0       59095840
>>> MutationStage                     0         0       31182942
>>> ReadRepairStage                   0         0            
 0
>>> ReplicateOnWriteStage            32    928042       28246716
>>> GossipStage                       1         1         449464
>>> AntiEntropyStage                  0         0            
 0
>>> MigrationStage                    0         0            
 4
>>> MemtablePostFlusher               0         0            445
>>> StreamStage                       0         0            
 0
>>> FlushWriter                       0         0            445
>>> FILEUTILS-DELETE-POOL             0         0             38
>>> MiscStage                         0         0          
   0
>>> FlushSorter                       0         0            
 0
>>> InternalResponseStage             0         0              4
>>> HintedHandoff                     0         0            
 0
>>>
>>> Second highest cpu usage one:
>>>
>>> 3 $ ./nodetool -h localhost tpstats
>>> Pool Name                    Active   Pending      Completed
>>> ReadStage                         0         0          
   0
>>> RequestResponseStage              0         0       27413910
>>> MutationStage                     0         0       27523094
>>> ReadRepairStage                   0         0            
 0
>>> ReplicateOnWriteStage             0         0       11111971
>>> GossipStage                       0         0         335422
>>> AntiEntropyStage                  0         0            
 0
>>> MigrationStage                    0         0            
 4
>>> MemtablePostFlusher               0         0             66
>>> StreamStage                       0         0            
 0
>>> FlushWriter                       0         0            
66
>>> FILEUTILS-DELETE-POOL             0         0             31
>>> MiscStage                         0         0          
   0
>>> FlushSorter                       0         0            
 0
>>> InternalResponseStage             0         0              4
>>> HintedHandoff                     0         0            
 0
>>>
>>> The other 2 that see minimal cpu usage:
>>>
>>> 2 $ ./nodetool -h localhost tpstats
>>> Pool Name                    Active   Pending      Completed
>>> ReadStage                         0         0          
   0
>>> RequestResponseStage              0         0        9213999
>>> MutationStage                     0         0       33233114
>>> ReadRepairStage                   0         0            
 0
>>> ReplicateOnWriteStage             0         0              0
>>> GossipStage                       0         0         336016
>>> AntiEntropyStage                  0         0            
 0
>>> MigrationStage                    0         0            
 4
>>> MemtablePostFlusher               0         0             77
>>> StreamStage                       0         0            
 0
>>> FlushWriter                       0         0            
77
>>> FILEUTILS-DELETE-POOL             0         0             40
>>> MiscStage                         0         0          
   0
>>> FlushSorter                       0         0            
 0
>>> InternalResponseStage             0         0              4
>>> HintedHandoff                     0         0            
 0
>>>
>>>
>>> 1 $ ./nodetool -h localhost tpstats
>>> Pool Name                    Active   Pending      Completed
>>> ReadStage                         0         0          
   0
>>> RequestResponseStage              0         0       13796875
>>> MutationStage                     0         0       36116662
>>> ReadRepairStage                   0         0            
 0
>>> ReplicateOnWriteStage             0         0              0
>>> GossipStage                       0         0         336496
>>> AntiEntropyStage                  0         0            
 0
>>> MigrationStage                    0         0            
 4
>>> MemtablePostFlusher               0         0             91
>>> StreamStage                       0         0            
 0
>>> FlushWriter                       0         0            
91
>>> FILEUTILS-DELETE-POOL             0         0             43
>>> MiscStage                         0         0          
   0
>>> FlushSorter                       0         0            
 0
>>> InternalResponseStage             0         0              4
>>> HintedHandoff                     0         0            
 0
>
>

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