more regionservers does not improve performance

[Jonathan Bishop <jb...@gmail.com>]

Hi,

I am running a MR job with 40 simultaneous mappers, each of which does puts
to HBase. I have ganged up the puts into groups of 1000 (this seems to help
quite a bit) and also made sure that the table is pre-split into 100
regions, and that the row keys are randomized using MD5 hashing.

My cluster size is 10, and I am allowing 4 mappers per tasktracker.

In my MR job I know that the mappers are able to generate puts much faster
than the puts can be handled in hbase. In other words if I let the mappers
run without doing hbase puts then everything scales as you would expect
with the number of mappers created. It is the hbase puts which seem to be
the bottleneck.

What is strange is that I do not get much run time improvement by
increasing the number regionservers beyond about 4. Indeed, it seems that
the system runs slower with 8 regionservers than with 4.

I have added the following in hbase-env.sh hoping this would help... (from
the book HBase in Action)

export HBASE_OPTS="-Xmx8g"
export HBASE_REGIONSERVER_OPTS="-Xmx8g -Xms8g -Xmn128m -XX:+UseParNewGC
-XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=70"

# Uncomment below to enable java garbage collection logging in the .out
file.
export HBASE_OPTS="${HBASE_OPTS} -verbose:gc -XX:+PrintGCDetails
-XX:+PrintGCDateStamps -Xloggc:${HBASE_HOME}/logs/gc-hbase.log"

Monitoring hbase through the web ui I see that there are pauses for
flushing, which seems to run pretty quickly, and for compacting, which
seems to take somewhat longer.

Any advice for making this run faster would be greatly appreciated.
Currently I am looking into installing Ganglia to better monitory my
cluster, but yet to have that running.

I suspect an I/O issue as the regionservers do not seem terribly loaded.

Thanks,

Jon

RE: more regionservers does not improve performance

[Pankaj Misra <pa...@impetus.co.in>]

Hi Jonathan,

What seems to me is that, while doing the split across all 40 mappers, the keys are not randomized enough to leverage multiple regions and the pre-split strategy. This may be happening because all the 40 mappers may be trying to write onto a single region for sometime, making it a HOT region,  till the key falls into another region, and then the other region becomes a HOT region hence you may seeing a high impact of compaction cycles reducing your throughput.

Are the keys incremental? Are the keys randomized enough across the splits?

Ideally when all 40 mappers are running you should see all the regions being filled up in parallel for maximum throughput. Hope it helps.

Thanks and Regards
Pankaj Misra


________________________________________
From: Jonathan Bishop [jbishop.rwc@gmail.com]
Sent: Friday, October 12, 2012 5:38 AM
To: user@hbase.apache.org
Subject: more regionservers does not improve performance

Hi,

I am running a MR job with 40 simultaneous mappers, each of which does puts
to HBase. I have ganged up the puts into groups of 1000 (this seems to help
quite a bit) and also made sure that the table is pre-split into 100
regions, and that the row keys are randomized using MD5 hashing.

My cluster size is 10, and I am allowing 4 mappers per tasktracker.

In my MR job I know that the mappers are able to generate puts much faster
than the puts can be handled in hbase. In other words if I let the mappers
run without doing hbase puts then everything scales as you would expect
with the number of mappers created. It is the hbase puts which seem to be
the bottleneck.

What is strange is that I do not get much run time improvement by
increasing the number regionservers beyond about 4. Indeed, it seems that
the system runs slower with 8 regionservers than with 4.

I have added the following in hbase-env.sh hoping this would help... (from
the book HBase in Action)

export HBASE_OPTS="-Xmx8g"
export HBASE_REGIONSERVER_OPTS="-Xmx8g -Xms8g -Xmn128m -XX:+UseParNewGC
-XX:+UseConcMarkSweepGC -XX:CMSInitiatingOccupancyFraction=70"

# Uncomment below to enable java garbage collection logging in the .out
file.
export HBASE_OPTS="${HBASE_OPTS} -verbose:gc -XX:+PrintGCDetails
-XX:+PrintGCDateStamps -Xloggc:${HBASE_HOME}/logs/gc-hbase.log"

Monitoring hbase through the web ui I see that there are pauses for
flushing, which seems to run pretty quickly, and for compacting, which
seems to take somewhat longer.

Any advice for making this run faster would be greatly appreciated.
Currently I am looking into installing Ganglia to better monitory my
cluster, but yet to have that running.

I suspect an I/O issue as the regionservers do not seem terribly loaded.

Thanks,

Jon

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