Cassandra read latency high even with row caching, why? - performance

Question

I am testing cassandra performance with a simple model.
CREATE TABLE "NoCache" (
key ascii,
column1 ascii,
value ascii,
PRIMARY KEY (key, column1)
) WITH COMPACT STORAGE AND
bloom_filter_fp_chance=0.010000 AND
caching='ALL' AND
comment='' AND
dclocal_read_repair_chance=0.000000 AND
gc_grace_seconds=864000 AND
read_repair_chance=0.100000 AND
replicate_on_write='true' AND
populate_io_cache_on_flush='false' AND
compaction={'class': 'SizeTieredCompactionStrategy'} AND
compression={'sstable_compression': 'SnappyCompressor'};
I am fetching 100 columns of a row key using pycassa, get/xget function (). but getting read latency about 15ms in the server.
colums=COL_FAM.get(row_key, column_count=100)
nodetool cfstats
Column Family: NoCache
SSTable count: 1
Space used (live): 103756053
Space used (total): 103756053
Number of Keys (estimate): 128
Memtable Columns Count: 0
Memtable Data Size: 0
Memtable Switch Count: 0
Read Count: 20
Read Latency: 15.717 ms.
Write Count: 0
Write Latency: NaN ms.
Pending Tasks: 0
Bloom Filter False Positives: 0
Bloom Filter False Ratio: 0.00000
Bloom Filter Space Used: 976
Compacted row minimum size: 4769
Compacted row maximum size: 557074610
Compacted row mean size: 87979499
Latency of this type is amazing! When nodetool info shows that read hits directly in the row cache.
Row Cache : size 4834713 (bytes), capacity 67108864 (bytes), 35 hits, 38 requests, 1.000 recent hit rate, 0 save period in seconds
Can anyone tell me why is cassandra taking so much time while reading from row cache?

Answer 1

Enable tracing and see what it's doing. http://www.datastax.com/dev/blog/tracing-in-cassandra-1-2