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From Jay Booth <jaybo...@gmail.com>
Subject Re: Slowdown with Hadoop Sort benchmark when using Jumbo frames?
Date Tue, 26 Jan 2010 19:39:02 GMT
Did you set io.file.buffer.size (or whatever the property is) to a large
value?  If you didn't, then hadoop will explicitly create packets at the
default size, and you'll see zero utilization of the jumboframes.  (although
that would argue for a flat performance, no?  do jumbo frames have increased
overhead-per-packet that could come into play with smaller packets here?)

Also, even if you do set that, I'm not 100% sure that it will lead to use of
the jumboframes, I'd have to take a look through the code to be sure.
 Another issue is that you may lose any networking performance gains as a
result of increased buffer allocation costs and garbage collection.

Ed, I threw out the radfs experiment, I'm working on HDFS-918 now -- that's
what I was mentioning the other night as being possibly more conducive to
jumbo frames due to less buffer allocation on BlockSender instantiation.
 But I haven't benchmarked yet so that's all very much "in theory"

On Tue, Jan 26, 2010 at 1:12 PM, stephen mulcahy

> Eli Collins wrote:
>> I was somewhat surprised at the results of enabling Jumbo frames - it
>>> resulted in a slowdown. In the case of the write operations, the slow
>>> down
>>> was about 5%. In the case of 10GB sort, the slowdown was around 6% and in
>>> the case of the 100GB sort, the slowdown was nearly 20%.
>> Are normal (non-hadoop) file transfers faster between the same hosts
>> with jumbo frames enabled? ie have your ruled out host/configuration
>> issues?
> Yes - I did some testing on two different clusters we have - both with and
> without jumbo frames and in both cases, I saw a small but noticeable
> increase in the overall bandwidth when moving from an MTU of 1500 to an MTU
> of 9000.
> Has anyone else tested Hadoop performance with Jumbo frames? Are you seeing
> something different to what we're seeing?
> Thanks,
> -stephen
> --
> Stephen Mulcahy, DI2, Digital Enterprise Research Institute,
> NUI Galway, IDA Business Park, Lower Dangan, Galway, Ireland
> http://di2.deri.ie    http://webstar.deri.ie    http://sindice.com

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