On Mon, Aug 2, 2010 at 4:28 PM, Jackob Carlsson
<[EMAIL PROTECTED]> wrote:
> Thanks Nick, but "in-memory" means a combiner can only be used over a single
> mapper?right?! Is there a way we use it for several mappers as well? Also
> what do you mean by "it may or may not run on a particular map attempt"?
> On Mon, Aug 2, 2010 at 5:43 PM, Nick Jones <[EMAIL PROTECTED]> wrote:
>> Hi Jackob,
>> A combiner acts a lot like a reduce step but it's executed on the mapper
>> with in-memory data. I've seen a reduction in job execution time by adding
>> one. The one caveat to keep in mind is that it may or may not run on a
>> particular map attempt.
>> On 8/2/2010 10:39 AM, Jackob Carlsson wrote:
>>> Hi everyone,
>>> Could anyone please help me to understand the function of combiner?
>>> Thanks in advance
> Is there a way we use it for several mappers as well?
No. That is the exact opposite goal of the combiner. It runs locally.
>it may or may not run on a particular map attempt
It only runs when certain thresholds in the framework are reached.