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From lars hofhansl <la...@apache.org>
Subject Re: experiencing high latency for few reads in HBase
Date Thu, 29 Aug 2013 04:29:32 GMT
A 1s SLA is tough in HBase (or any large memory JVM application).

Maybe, if you presplit your table, play with JDK7 and the G1 collector, but nobody here will
vouch for such an SLA in the 99th percentile.
I heard some folks have experimented with 30GB heaps and G1 and have reported max GC times
of 200ms, but I have not verified that.

-- Lars

----- Original Message -----
From: Saurabh Yahoo <saurabh_ag@yahoo.com>
To: "user@hbase.apache.org" <user@hbase.apache.org>
Cc: "user@hbase.apache.org" <user@hbase.apache.org>
Sent: Wednesday, August 28, 2013 3:17 PM
Subject: Re: experiencing high latency for few reads in HBase 

Hi Vlad,

Thanks for your response.

1. Our SLA is less than one sec. we cannot afford latency more than 1 sec. 

We can increase heap size if that help, we have enough memory on server. What would be the
optimal heap size? 

2. Cache hit ratio is 95%.  One thing I don't understand that we have allocated only 4gb
for block cache out of 12gb. That left 8gb for rest of JVM. There is no write. Memcache is
empty. Is 8gb not enough for hbase to process the requests? What are the most memory consuming
objects in region server? 

3. We will change the cf to IN_memory and report back performance difference.  


On Aug 28, 2013, at 3:15 PM, Vladimir Rodionov <vrodionov@carrieriq.com> wrote:

> 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: vrodionov@carrieriq.com
> ________________________________________
> From: Saurabh Yahoo [saurabh_ag@yahoo.com]
> Sent: Wednesday, August 28, 2013 5:10 AM
> To: user@hbase.apache.org
> 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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