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Hadoop >> mail # user >> Suggestions for swapping issue

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Re: Suggestions for swapping issue

On May 11, 2011, at 11:11 AM, Adi wrote:

> By our calculations hadoop should not exceed 70% of memory.
> Allocated per node - 48 map slots (24 GB) ,  12 reduce slots (6 GB), 1 GB
> each for DataNode/TaskTracker and one JobTracker Totalling 33/34 GB
> allocation.

It sounds like you are only taking into consideration the heap size.  There is more memory allocated than just the heap...

> The queues are capped at using only 90% of capacity allocated so generally
> 10% of slots are always kept free.

But that doesn't translate into how free the nodes, which you've discovered.  Individual nodes should be configured based on the assumption that *all* slots will be used.

> The cluster was running total 33 mappers and 1 reducer so around 8-9 mappers
> per node with 3 GB max limit and they were utilizing around 2GB each.
> Top was showing 100% memory utilized. Which our sys admin says is ok as the
> memory is used for file caching by linux if the processes are not using it.

Well, yes and no.  What is the breakdown of that 100%?  Is there any actually allocated to buffer cache or is it all user space?

> No swapping on 3 nodes.
> Then node4 just started swapping after the number of processes shot up
> unexpectedly. The main mystery are these excess number of processes on the
> node which went down. 36 as opposed to expected 11. The other 3 nodes were
> successfully executing the mappers without any memory/swap issues.

Likely speculative execution or something else.  But again: don't build machines with the assumption that only x% of the slots will get used.  There is no guarantee in the system that says that free slots will be balanced across all nodes... esp when you take into consideration node failure.

> -Adi
> On Wed, May 11, 2011 at 1:40 PM, Michel Segel <[EMAIL PROTECTED]>wrote:
>> You have to do the math...
>> If you have 2gb per mapper, and run 10 mappers per node... That means 20gb
>> of memory.
>> Then you have TT and DN running which also take memory...
>> What did you set as the number of mappers/reducers per node?
>> What do you see in ganglia or when you run top?
>> Sent from a remote device. Please excuse any typos...
>> Mike Segel
>> On May 11, 2011, at 12:31 PM, Adi <[EMAIL PROTECTED]> wrote:
>>> Hello Hadoop Gurus,
>>> We are running a 4-node cluster. We just upgraded the RAM to 48 GB. We
>> have
>>> allocated around 33-34 GB per node for hadoop processes. Leaving the rest
>> of
>>> the 14-15 GB memory for OS and as buffer. There are no other processes
>>> running on these nodes.
>>> Most of the lighter jobs run successfully but one big job is
>> de-stabilizing
>>> the cluster. One node starts swapping and runs out of swap space and goes
>>> offline. We tracked the processes on that node and noticed that it ends
>> up
>>> with more than expected hadoop-java processes.
>>> The other 3 nodes were running 10 or 11 processes and this node ends up
>> with
>>> 36. After killing the job we find these processes still show up and we
>> have
>>> to kill them manually.
>>> We have tried reducing the swappiness to 6 but saw the same results. It
>> also
>>> looks like hadoop stays well within the memory limits allocated and still
>>> starts swapping.
>>> Some other suggestions we have seen are:
>>> 1) Increase swap size. Current size is 6 GB. The most quoted size is
>> 'tons
>>> of swap' but note sure how much it translates to in numbers. Should it be
>> 16
>>> or 24 GB
>>> 2) Increase overcommit ratio. Not sure if this helps as a few blog
>> comments
>>> mentioned it didn't help
>>> Any other hadoop or linux config suggestions are welcome.
>>> Thanks.
>>> -Adi