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Re: experiencing high latency for few reads in HBase
Right 4 sec is good.  
@Saurabh - so your read is - getting 20 out of 25 millions rows ?. Is this a Get or a Scan ?
BTW, in this stress test how many concurrent clients do you have ? 
 
Regards,
- kiru
________________________________
 From: Vladimir Rodionov <[EMAIL PROTECTED]>
To: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
Sent: Wednesday, August 28, 2013 12:15 PM
Subject: RE: experiencing high latency for few reads in HBase
 

1. 4 sec max latency is not that bad taking into account 12GB heap.  It can be much larger. What is your SLA?
2. Block evictions is the result of a poor cache hit rate and the root cause of a periodic stop-the-world GC pauses (max latencies
    latencies you have been observing in the test)
3. Block cache consists of 3 parts (25% young generation, 50% - tenured, 25% - permanent). Permanent part is for CF with
IN_MEMORY = true (you can specify this when you create CF).  Block first stored in 'young gen' space, then gets promoted to 'tenured gen' space
(or gets evicted). May be your 'perm gen' space is underutilized? This is exact 25% of 4GB (1GB). Although HBase LruBlockCache should use all the space allocated for block cache -
there is no guarantee (as usual). If you don have in_memory column families you may decrease

Best regards,
Vladimir Rodionov
Principal Platform Engineer
Carrier IQ, www.carrieriq.com
e-mail: [EMAIL PROTECTED]

________________________________________
From: Saurabh Yahoo [[EMAIL PROTECTED]]
Sent: Wednesday, August 28, 2013 5:10 AM
To: [EMAIL PROTECTED]
Subject: experiencing high latency for few reads in HBase

Hi,

We are running a stress test in our 5 node cluster and we are getting the expected mean latency of 10ms. But we are seeing around 20 reads out of 25 million reads having latency more than 4 seconds. Can anyone provide the insight what we can do to meet below second SLA for each and every read?

We observe the following things -

1. Reads are evenly distributed among 5 nodes.  CPUs remain under 5% utilized.

2. We have 4gb block cache (30% block cache out of 12gb) setup. 3gb block cache got filled up but around 1gb remained free. There are a large number of cache eviction.

Questions to experts -

1. If there are still 1gb of free block cache available, why is hbase evicting the block from cache?

4. We are seeing memory went up to 10gb three times before dropping sharply to 5gb.

Any help is highly appreciable,

Thanks,
Saurabh.

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