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From Todd Burruss <bburr...@expedia.com>
Subject Re: Key cache hit rate issue
Date Fri, 17 Feb 2012 17:47:12 GMT
ah, I missed the part about "key" cache .. I read "row" cache.  thx

On 2/16/12 6:14 PM, "Jonathan Ellis" <jbellis@gmail.com> wrote:

>Look for this code in SSTableReader.getPosition:
>
>            Pair<Descriptor, DecoratedKey> unifiedKey = new
>Pair<Descriptor, DecoratedKey>(descriptor, decoratedKey);
>            Long cachedPosition = getCachedPosition(unifiedKey, true);
>
>On Thu, Feb 16, 2012 at 4:23 PM, Todd Burruss <bburruss@expedia.com>
>wrote:
>> jonathan, you said the key to the cache is key + sstable?  looking at
>>the
>> code it looks like a DecoratedKey is the "row key".  how does sstable
>>come
>> into play?
>>
>> On 2/16/12 1:20 PM, "Jonathan Ellis" <jbellis@gmail.com> wrote:
>>
>>>So, you have roughly 1/6 of your (physical) row keys cached and about
>>>1/4 cache hit rate, which doesn't sound unreasonable to me.  Remember,
>>>each logical key may be spread across multiple physical sstables --
>>>each (key, sstable) pair is one entry in the key cache.
>>>
>>>On Thu, Feb 16, 2012 at 1:48 PM, Eran Chinthaka Withana
>>><eran.chinthaka@gmail.com> wrote:
>>>> Hi Aaron,
>>>>
>>>> Here it is.
>>>>
>>>> Keyspace: XXXX
>>>> Read Count: 1123637972
>>>> Read Latency: 5.757938114343114 ms.
>>>> Write Count: 128201833
>>>> Write Latency: 0.0682576607387509 ms.
>>>> Pending Tasks: 0
>>>> Column Family: YY
>>>> SSTable count: 18
>>>> Space used (live): 103318720685
>>>> Space used (total): 103318720685
>>>> Number of Keys (estimate): 92404992
>>>> Memtable Columns Count: 1425580
>>>> Memtable Data Size: 359655747
>>>> Memtable Switch Count: 2522
>>>> Read Count: 1123637972
>>>> Read Latency: 14.731 ms.
>>>> Write Count: 128201833
>>>> Write Latency: NaN ms.
>>>> Pending Tasks: 0
>>>> Bloom Filter False Postives: 1488
>>>> Bloom Filter False Ratio: 0.00000
>>>> Bloom Filter Space Used: 331522920
>>>> Key cache capacity: 16637958
>>>> Key cache size: 16637958
>>>> Key cache hit rate: 0.2708333333333333
>>>> Row cache: disabled
>>>> Compacted row minimum size: 51
>>>> Compacted row maximum size: 6866
>>>> Compacted row mean size: 2560
>>>>
>>>> Thanks,
>>>> Eran Chinthaka Withana
>>>>
>>>>
>>>>
>>>> On Thu, Feb 16, 2012 at 12:30 AM, aaron morton
>>>><aaron@thelastpickle.com>
>>>> wrote:
>>>>>
>>>>> Its in the order of 261 to 8000 and the ratio is 0.00. But i guess
>>>>>8000 is
>>>>> bit high. Is there a way to fix/improve it?
>>>>>
>>>>> Sorry I don't understand what you mean. But if the ratio is 0.0 all
>>>>>is
>>>>> good.
>>>>>
>>>>> Could you include the full output from cfstats for the CF you are
>>>>>looking
>>>>> at ?
>>>>>
>>>>> Cheers
>>>>>
>>>>> -----------------
>>>>> Aaron Morton
>>>>> Freelance Developer
>>>>> @aaronmorton
>>>>> http://www.thelastpickle.com
>>>>>
>>>>> On 15/02/2012, at 1:00 PM, Eran Chinthaka Withana wrote:
>>>>>
>>>>> Its in the order of 261 to 8000 and the ratio is 0.00. But i guess
>>>>>8000 is
>>>>> bit high. Is there a way to fix/improve it?
>>>>>
>>>>> Thanks,
>>>>> Eran Chinthaka Withana
>>>>>
>>>>>
>>>>> On Tue, Feb 14, 2012 at 3:42 PM, aaron morton
>>>>><aaron@thelastpickle.com>
>>>>> wrote:
>>>>>>
>>>>>> Out of interest what does cfstats say about the bloom filter stats
>>>>>>? A
>>>>>> high false positive could lead to a low key cache hit rate.
>>>>>>
>>>>>> Also, is there a way to warm start the key cache, meaning pre-load
>>>>>>the
>>>>>> amount of keys I set as keys_cached?
>>>>>>
>>>>>> See key_cache_save_period when creating the CF.
>>>>>>
>>>>>> Cheers
>>>>>>
>>>>>>
>>>>>> -----------------
>>>>>> Aaron Morton
>>>>>> Freelance Developer
>>>>>> @aaronmorton
>>>>>> http://www.thelastpickle.com
>>>>>>
>>>>>> On 15/02/2012, at 5:54 AM, Eran Chinthaka Withana wrote:
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> I'm using Cassandra 1.0.7 and I've set the keys_cached to about 80%
>>>>>> (using the numerical values). This is visible in cfstats too. But
>>>>>>I'm
>>>>>> getting less than 20% (or sometimes even 0%) key cache hit rate.
>>>>>>Well, the
>>>>>> data access pattern is not the issue here as I know they are
>>>>>>retrieving the
>>>>>> same row multiple times. I'm using hector client with dynamic load
>>>>>>balancing
>>>>>> policy with consistency ONE for both reads and writes. Any ideas
on
>>>>>>how to
>>>>>> find the issue and fix this?
>>>>>>
>>>>>> Here is what I see on cfstats.
>>>>>>
>>>>>> Key cache capacity: 16637958
>>>>>> Key cache size: 16637958
>>>>>> Key cache hit rate: 0.045454545454545456
>>>>>>
>>>>>> Also, is there a way to warm start the key cache, meaning pre-load
>>>>>>the
>>>>>> amount of keys I set as keys_cached?
>>>>>>
>>>>>> Thanks,
>>>>>> Eran
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>
>>>
>>>
>>>
>>>--
>>>Jonathan Ellis
>>>Project Chair, Apache Cassandra
>>>co-founder of DataStax, the source for professional Cassandra support
>>>http://www.datastax.com
>>
>
>
>
>-- 
>Jonathan Ellis
>Project Chair, Apache Cassandra
>co-founder of DataStax, the source for professional Cassandra support
>http://www.datastax.com


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