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From Alain RODRIGUEZ <arodr...@gmail.com>
Subject Re: Heap size question
Date Tue, 21 Aug 2012 15:43:30 GMT
You're welcome. I'll answer to your new questions but keep in mind that I
am not a cassandra commiter nor even a cassandra specialist.

"you mean that key cache is not in heap? I am using cassandra 1.0.8 and I
was under the expression it was, see
http://www.datastax.com/docs/1.0/operations/tuning, Tuning Java Heap Size."

http://www.datastax.com/dev/blog/whats-new-in-cassandra-1-0-improved-memory-and-disk-space-management

If I understood this correctly, It seems that  only the row cache is
off-heap. So it's not an issue for us as far as we don't use row cache.

"I thought that key-cache-size + 1GB + memtable space should not exceed
heap size. Am I wrong?"

I don't know if this is a good formula. Datastax gives it so it shouldn't
be that bad :). However I would say that "key-cache-size + 1GB + memtable
space"  should not exceed 0.75 * Max Heap (where 0.75 is
flush_largest_memtables_at). I keep default key-cache (which is 5% of max
heap if I remember well on 1.1.x) and default memtable space (1/3 of max
heap). I have enlarged my heap from 2 to 4 GB because I had some memory
pressure (sometimes the Heap Used was greater than 0.75 * Max Heap)

"WARN [ScheduledTasks:1] 2012-08-20 12:31:46,506 GCInspector.java (line
145) Heap is 0.7704251937535934 full.  You may need to reduce memtable
and/or cache sizes.  Cassandra will now flush up to the two largest
memtables to free up memory.  Adjust flush_largest_memtables_at threshold
in cassandra.yaml if you don't want Cassandra to do this automatically"

This message is the memory pressure I was talking about just above.

"How do I know if my off-heap memory is not used?"

Well, if you got no row cache and your server is only used as a Cassandra
node, I'm quite sure you can tune your heap to get 4GB. I guess a htop or
any memory monitoring system is able to tell you how much your memory is
used.

I hope I didn't tell you too much bullshits :p.

Alain

2012/8/21 Tamar Fraenkel <tamar@tok-media.com>

> Thanks for you prompt response. Please see follow up questions below
> Thanks!!!
>
>
>
> *Tamar Fraenkel *
> Senior Software Engineer, TOK Media
>
> [image: Inline image 1]
>
> tamar@tok-media.com
> Tel:   +972 2 6409736
> Mob:  +972 54 8356490
> Fax:   +972 2 5612956
>
>
>
>
>
> On Tue, Aug 21, 2012 at 12:57 PM, Alain RODRIGUEZ <arodrime@gmail.com>wrote:
>
>> I have the same configuration and I recently change  my cassandra-sh.yaml
>> to :
>>
>> MAX_HEAP_SIZE="4G"
>> HEAP_NEWSIZE="200M"
>>
>
>> I guess it depends on how much you use the cache (which is now in the
>> off-heap memory).
>>
>
>  you mean that key cache is not in heap? I am using cassandra 1.0.8 and I
> was under the expression it was, see
> http://www.datastax.com/docs/1.0/operations/tuning, Tuning Java Heap Size.
>  I thought that key-cache-size + 1GB + memtable space should not exceed
> heap size. Am I wrong?
>
>
>> I don't use row cache and use the default key cache size.
>>
> Me too, I have Key Cache capacity of 200000 for all my CFs. Currently if
> my calculations are correct I have about 1.4GB of key cache.
>
>>
>> I have no more memory pressure nor OOM.
>>
> I don't see OOM, but I do see messages like the following in my logs:
> INFO [ScheduledTasks:1] 2012-08-20 12:31:46,506 GCInspector.java (line
> 122) GC for ParNew: 219 ms for 1 collections, 1491982816 used; max is
> 1937768448
>  WARN [ScheduledTasks:1] 2012-08-20 12:31:46,506 GCInspector.java (line
> 145) Heap is 0.7704251937535934 full.  You may need to reduce memtable
> and/or cache sizes.  Cassandra will now flush up to the two largest
> memtables to free up memory.  Adjust flush_largest_memtables_at threshold
> in cassandra.yaml if you don't want Cassandra to do this automatically
>
>
>
>> I think that if your off-heap memory is unused, it's better enlarging the
>> heap (with a max limit of 8GB)
>>
>> How do I know if my off-heap memory is not used?
>
>
>> Hope this will help.
>>
>> Alain
>>
>> 2012/8/21 Tamar Fraenkel <tamar@tok-media.com>
>>
>>> Hi!
>>> I have a question regarding Cassandra heap size.
>>> Cassandra calculates heap size in cassandra-env.sh according to the
>>> following algorythm
>>>     # set max heap size based on the following
>>>     # max(min(1/2 ram, 1024MB), min(1/4 ram, 8GB))
>>>     # calculate 1/2 ram and cap to 1024MB
>>>     # calculate 1/4 ram and cap to 8192MB
>>>     # pick the max
>>>
>>> So, for
>>> system_memory_in_mb=7468
>>> half_system_memory_in_mb=3734
>>> quarter_system_memory_in_mb=1867
>>> This will result in
>>> max(min(3734,1024), min(1867,8000)) = max(1024,1867)=*1867MB* or in
>>> other words 1/4 of RAM.
>>>
>>> In http://www.datastax.com/docs/1.0/operations/tuning it says: "Cassandra's
>>> default configuration opens the JVM with a heap size of 1/4 of the
>>> available system memory (or a minimum 1GB and maximum of 8GB for systems
>>> with a very low or very high amount of RAM). Heapspace should be a minimum
>>> of 1/2 of your RAM, but a maximum of 8GB. The vast majority of deployments
>>> do not benefit from larger heap sizes because (in most cases) the ability
>>> of Java 6 to gracefully handle garbage collection above 8GB quickly
>>> diminishes."
>>> *If I understand this correctly, this means it is better if my heap
>>> size will be 1/2 of RAM, 3734MB.*
>>> I am running on EC2 m1.large instance (7.5 GB memory, 4 EC2 Compute
>>> Units (2 virtual cores with 2 EC2 Compute Units each)).
>>> My system seems to be suffering from lack of memory, and I should
>>> probably increase heap or (and?) reduce key cache size.
>>>
>>> Would you recommend changing the heap to half RAM?
>>>
>>> If yes, should I hard-code it in acassandra-env.sh?
>>>
>>> Thanks!
>>>
>>> *Tamar Fraenkel *
>>> Senior Software Engineer, TOK Media
>>>
>>> [image: Inline image 1]
>>>
>>> tamar@tok-media.com
>>> Tel:   +972 2 6409736
>>> Mob:  +972 54 8356490
>>> Fax:   +972 2 5612956
>>>
>>>
>>>
>>>
>>
>

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