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From "Liu, Raymond" <raymond....@intel.com>
Subject One weird problem of my MR job upon hbase table.
Date Sat, 05 Jan 2013 03:37:11 GMT

I encounter a weird lag behind map task issue here :

I have a small hadoop/hbase cluster with 1 master node and 4 regionserver node all have 16
CPU with map and reduce slot set to 24.

A few table is created with regions distributed on each region node evenly ( say 16 region
for each region server). Also each region has almost the same number of kvs with very similar
size. All table had major_compact done to ensure data locality

I have a MR job which simply do local region scan in every map task ( so 16 map task for each
regionserver node).

By theory, every map task should finish within similar time.

But the real case is that some regions on the same region server always lags behind a lot,
say cost 150 ~250% of the other map tasks average times.

If this is happen to a single region server for every table, I might doubt it is a disk issue
or other reason that bring down the performance of this region server.

But the weird thing is that, though with each single table, almost all the map task on the
the same single regionserver is lag behind. But for different table, this lag behind regionserver
is different! And the region and region size is distributed evenly which I double checked
for a lot of times. ( I even try to set replica to 4 to ensure every node have a copy of local

Say table 1, all map task on regionserver node 2 is slow. While for table 2, maybe all map
task on regionserver node 3 is slow, and with table 1, it will always be regionserver node
2 which is slow regardless of cluster restart, and the slowest map task will always be the
very same one. And it won't go away even I do major compact again.....

So, anyone could give me some clue on what reason might possible lead to this weird behavior?
Any wild guess is welcome! 

(BTW. I don't encounter this issue a few days ago with the same table. While I do restart
cluster and do a few changes upon config file during that period, But restore the config file
don't help)

Best Regards,
Raymond Liu

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