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From Bryan Keller <brya...@gmail.com>
Subject Re: Poor HBase map-reduce scan performance
Date Wed, 01 May 2013 06:02:25 GMT
The table has hashed keys so rows are evenly distributed amongst the regionservers, and load
on each regionserver is pretty much the same. I also have per-table balancing turned on. I
get mostly data local mappers with only a few rack local (maybe 10 of the 250 mappers).

Currently the table is a wide table schema, with lists of data structures stored as columns
with column prefixes grouping the data structures (e.g. 1_name, 1_address, 1_city, 2_name,
2_address, 2_city). I was thinking of moving those data structures to protobuf which would
cut down on the number of columns. The downside is I can't filter on one value with that,
but it is a tradeoff I would make for performance. I was also considering restructuring the
table into a tall table.

Something interesting is that my old regionserver machines had five 15k SCSI drives instead
of 2 SSDs, and performance was about the same. Also, my old network was 1gbit, now it is 10gbit.
So neither network nor disk I/O appear to be the bottleneck. The CPU is rather high for the
regionserver so it seems like the best candidate to investigate. I will try profiling it tomorrow
and will report back. I may revisit compression on vs off since that is adding load to the
CPU.

I'll also come up with a sample program that generates data similar to my table.


On Apr 30, 2013, at 10:01 PM, lars hofhansl <larsh@apache.org> wrote:

> Your average row is 35k so scanner caching would not make a huge difference, although
I would have expected some improvements by setting it to 10 or 50 since you have a wide 10ge
pipe.
> 
> I assume your table is split sufficiently to touch all RegionServer... Do you see the
same load/IO on all region servers?
> 
> A bunch of scan improvements went into HBase since 0.94.2.
> I blogged about some of these changes here: http://hadoop-hbase.blogspot.com/2012/12/hbase-profiling.html
> 
> In your case - since you have many columns, each of which carry the rowkey - you might
benefit a lot from HBASE-7279.
> 
> In the end HBase *is* slower than straight HDFS for full scans. How could it not be?
> So I would start by looking at HDFS first. Make sure Nagle's is disbaled in both HBase
and HDFS.
> 
> And lastly SSDs are somewhat new territory for HBase. Maybe Andy Purtell is listening,
I think he did some tests with HBase on SSDs.
> With rotating media you typically see an improvement with compression. With SSDs the
added CPU needed for decompression might outweigh the benefits.
> 
> At the risk of starting a larger discussion here, I would posit that HBase's LSM based
design, which trades random IO with sequential IO, might be a bit more questionable on SSDs.
> 
> If you can, it would be nice to run a profiler against one of the RegionServers (or maybe
do it with the single RS setup) and see where it is bottlenecked.
> (And if you send me a sample program to generate some data - not 700g, though :) - I'll
try to do a bit of profiling during the next days as my day job permits, but I do not have
any machines with SSDs).
> 
> -- Lars
> 
> 
> 
> 
> ________________________________
> From: Bryan Keller <bryanck@gmail.com>
> To: user@hbase.apache.org 
> Sent: Tuesday, April 30, 2013 9:31 PM
> Subject: Re: Poor HBase map-reduce scan performance
> 
> 
> Yes, I have tried various settings for setCaching() and I have setCacheBlocks(false)
> 
> On Apr 30, 2013, at 9:17 PM, Ted Yu <yuzhihong@gmail.com> wrote:
> 
>> From http://hbase.apache.org/book.html#mapreduce.example :
>> 
>> scan.setCaching(500);        // 1 is the default in Scan, which will
>> be bad for MapReduce jobs
>> scan.setCacheBlocks(false);  // don't set to true for MR jobs
>> 
>> I guess you have used the above setting.
>> 
>> 0.94.x releases are compatible. Have you considered upgrading to, say
>> 0.94.7 which was recently released ?
>> 
>> Cheers
>> 
>> On Tue, Apr 30, 2013 at 9:01 PM, Bryan Keller <bryanck@gmail.com> wrote:
>> 
>>> I have been attempting to speed up my HBase map-reduce scans for a while
>>> now. I have tried just about everything without much luck. I'm running out
>>> of ideas and was hoping for some suggestions. This is HBase 0.94.2 and
>>> Hadoop 2.0.0 (CDH4.2.1).
>>> 
>>> The table I'm scanning:
>>> 20 mil rows
>>> Hundreds of columns/row
>>> Column keys can be 30-40 bytes
>>> Column values are generally not large, 1k would be on the large side
>>> 250 regions
>>> Snappy compression
>>> 8gb region size
>>> 512mb memstore flush
>>> 128k block size
>>> 700gb of data on HDFS
>>> 
>>> My cluster has 8 datanodes which are also regionservers. Each has 8 cores
>>> (16 HT), 64gb RAM, and 2 SSDs. The network is 10gbit. I have a separate
>>> machine acting as namenode, HMaster, and zookeeper (single instance). I
>>> have disk local reads turned on.
>>> 
>>> I'm seeing around 5 gbit/sec on average network IO. Each disk is getting
>>> 400mb/sec read IO. Theoretically I could get 400mb/sec * 16 = 6.4gb/sec.
>>> 
>>> Using Hadoop's TestDFSIO tool, I'm seeing around 1.4gb/sec read speed. Not
>>> really that great compared to the theoretical I/O. However this is far
>>> better than I am seeing with HBase map-reduce scans of my table.
>>> 
>>> I have a simple no-op map-only job (using TableInputFormat) that scans the
>>> table and does nothing with data. This takes 45 minutes. That's about
>>> 260mb/sec read speed. This is over 5x slower than straight HDFS.
>>> Basically, with HBase I'm seeing read performance of my 16 SSD cluster
>>> performing nearly 35% slower than a single SSD.
>>> 
>>> Here are some things I have changed to no avail:
>>> Scan caching values
>>> HDFS block sizes
>>> HBase block sizes
>>> Region file sizes
>>> Memory settings
>>> GC settings
>>> Number of mappers/node
>>> Compressed vs not compressed
>>> 
>>> One thing I notice is that the regionserver is using quite a bit of CPU
>>> during the map reduce job. When dumping the jstack of the process, it seems
>>> like it is usually in some type of memory allocation or decompression
>>> routine which didn't seem abnormal.
>>> 
>>> I can't seem to pinpoint the bottleneck. CPU use by the regionserver is
>>> high but not maxed out. Disk I/O and network I/O are low, IO wait is low.
>>> I'm on the verge of just writing the dataset out to sequence files once a
>>> day for scan purposes. Is that what others are doing?


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