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From Thomas Spengler <thomas.speng...@toptarif.de>
Subject Re: Heap size question
Date Wed, 22 Aug 2012 06:23:39 GMT
Thats not right

since 1.1.X it will used

my hint:
take a look at

commitlog_total_space_in_mb
it seams to be maped to off heap usage
the fix for taking the whole off-heap-memory is included in 1.1.3

the next parameter you have to take a look

disk_access_mode: mmap_index_only
#disk_access_mode: auto
#disk_access_mode: standard

and the third

row_cache_provider:

the default has changed from 1.0.X to 1.1.X
the new one takes off-heap-cache (with and also without jna)

Regards
Tom

On 08/22/2012 06:37 AM, aaron morton wrote:
>> "How do I know if my off-heap memory is not used?"
> 
> 
> If you are using the default memory mapped file access memory not used by the cassandra
JVM will be used to cache files. 
> 
> Cheers
> 
> -----------------
> Aaron Morton
> Freelance Developer
> @aaronmorton
> http://www.thelastpickle.com
> 
> On 22/08/2012, at 5:17 AM, Tamar Fraenkel <tamar@tok-media.com> wrote:
> 
>> Much appreciated.
>> What you described makes a lot of sense from all my readings :)
>>
>> Thanks!
>> Tamar Fraenkel 
>> Senior Software Engineer, TOK Media 
>>
>> <tokLogo.png>
>>
>> tamar@tok-media.com
>> Tel:   +972 2 6409736 
>> Mob:  +972 54 8356490 
>> Fax:   +972 2 5612956 
>>
>>
>>
>>
>>
>> On Tue, Aug 21, 2012 at 6:43 PM, Alain RODRIGUEZ <arodrime@gmail.com> wrote:
>> 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 
>>
>> <tokLogo.png>
>>
>> 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 
>>
>> <tokLogo.png>
>>
>> tamar@tok-media.com
>> Tel:   +972 2 6409736 
>> Mob:  +972 54 8356490 
>> Fax:   +972 2 5612956 
>>
>>
>>
>>
>>
>>
>>
> 
> 


-- 
Thomas Spengler
Chief Technology Officer
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