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From Shu Zhang <>
Subject RE: OOM on heavy write load
Date Mon, 25 Apr 2011 19:55:23 GMT
How large are your rows? binary_memtable_throughput_in_
mb only tracks size of data, but there is an overhead associated with each row on the order
of magnitude of a few KBs. If your row data sizes are really small then the overhead dominates
the memory usage and binary_memtable_throughput_in_
mb end up not limiting your memory usage the way you'd expect. It's a good idea to specify
memtable_operations_in_millions in that case. If you're not sure how big your data is compared
to memtable overhead, it's a good idea to specify both parameters to effectively put in a
memory ceiling no matter which dominates your actual memory usage.

It could also be that your key cache is too big, you should measure your key sizes and make
sure you have enough memory to cache 1m keys (along with your memtables). Finally if you have
multiple keyspaces (for multiple applications) on your cluster, they all share the total available
heap, so you have to account for that.

There's no measure in cassandra to guard against OOM, you must configure nodes such that the
max memory usage on each node, that is max size all your caches and memtables can potentially
grow to, is less than your heap size.
From: Nikolay Kоvshov []
Sent: Monday, April 25, 2011 5:21 AM
Subject: Re: OOM on heavy write load

I assume if I turn off swap it will just die earlier, no ? What is the mechanism of dying

>From the link you provided

# Row cache is too large, or is caching large rows
my row_cache is 0

# The memtable sizes are too large for the amount of heap allocated to the JVM
Is my memtable size too large ? I have made it less to surely fit the "magical formula"

Trying to analyze heap dumps gives me the following:

In one case diagram has 3 Memtables about 64 Mb each + 72 Mb "Thread" + 700 Mb "Unreachable

suspected threats:
7 instances of "org.apache.cassandra.db.Memtable", loaded by "sun.misc.Launcher$AppClassLoader
@ 0x7f29f4992d68" occupy 456,292,912 (48.36%) bytes.
25,211 instances of "", loaded by "sun.misc.Launcher$AppClassLoader
@ 0x7f29f4992d68" occupy 294,908,984 (31.26%) byte
72 instances of "java.lang.Thread", loaded by "<system class loader>" occupy 143,632,624
(15.22%) bytes.

In other cases memory analyzer hangs trying to parse 2Gb dump

22.04.2011, 17:26, "Jonathan Ellis" <>;:

>  (0) turn off swap
>  (1)
>  On Fri, Apr 22, 2011 at 8:00 AM, Nikolay Kоvshov <>;; wrote:
>>   I am using Cassandra 0.7.0 with following settings
>>   binary_memtable_throughput_in_mb: 64
>>   in_memory_compaction_limit_in_mb: 64
>>   keys_cached 1 million
>>   rows_cached 0
>>   RAM for Cassandra 2 GB
>>   I run very simple test
>>   1 Node with 4 HDDs (1 HDD - commitlog and caches, 3 HDDs - data)
>>   1 KS => 1 CF => 1 Column
>>   I insert data (random key 64 bytes + value 64 bytes) at a maximum possible speed,
trying to hit disk i/o, calculate speed and make sure Cassandra stays alive. It doesn't, unfortunately.
>>   After several hundreds millions of inserts Cassandra always does down by OOM. Getting
it up again doesn't help - after inserting some new data it goes down again. By this time
Cassandra goes to swap and has a lot of tasks pending. I am not inserting anything now and
tasks sloooowly disappear, but it will take her weeks to do all of them.
>>   compaction type: Minor
>>   column family: Standard1
>>   bytes compacted: 3661003227
>>   bytes total in progress: 4176296448
>>   pending tasks: 630
>>   So, what am I (or Cassandra) doing wrong ? I dont want to get Cassandra crashed
without means of repair on heavy load circumstances.
>  --
>  Jonathan Ellis
>  Project Chair, Apache Cassandra
>  co-founder of DataStax, the source for professional Cassandra support

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