> "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 <ta...@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> > > ta...@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 <arodr...@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 <ta...@tok-media.com> > Thanks for you prompt response. Please see follow up questions below > Thanks!!! > > > > Tamar Fraenkel > Senior Software Engineer, TOK Media > > <tokLogo.png> > > ta...@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 <arodr...@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 <ta...@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> > > ta...@tok-media.com > Tel: +972 2 6409736 > Mob: +972 54 8356490 > Fax: +972 2 5612956 > > > > > > >
