If you are using CombineHiveInputFormat it might be the case that all files are being combined into one large split and hence 1 mapper gets created.
If that is the case you can set the max split size in hive-default.xml config file to create more splits and hence more map tasks:
<description>The maximum size chunk that map input should be split
From: Edward Capriolo
Sent: Thursday, July 28, 2011 7:10 AM
Subject: Re: Hive 0.7 using only one mapper
On Thu, Jul 28, 2011 at 9:23 AM, Wojciech Langiewicz wrote:
I'm having isssue running Hive jobs after updating from Hive 0.5 to Hive 0.7 (from CDHb4 to CDHu1).
No matter what query I'm running Hive is always using one mapper.
I have tried different queries with various sizes of input and ones with many reducers or no reducers.
For version 0.5 everything worked correctly.
I'm attaching my hive-site.xml: https://gist.github.com/1111531
I have tested also jobs with Pig, and those jobs use multiple mappers - so I guess this is a Hive issue.
Thank you for all your help.
You should also check that your hive-default.xml and other conf/ files is up to 0.7.X. Having older versions of that file can lead to problems.