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https://issues.apache.org/jira/browse/HADOOP-2144?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12539665
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Raghu Angadi commented on HADOOP-2144:
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A few initial observations :
With 4 clients, it is cpu bound. Pretty much no cpu is left.. so disk read b/w
being lower is ok.
Assuming 'top' correctly adds up the % cpu of each of the threads for each cpu,
there is 400% cpu on the machine (not sure if this is 4 cores or 2 cpus with 2
hyper threads each).
DataNode takes : 180% on 4 threads
4 Clients take : 200-205% on 4 processes.
So DataNode does not eat up more cpu than the client. The main difference is
that Datanode has extra work to read from disk/kernel into user space and
client does checksum verification. So checksum verification takes a bit more
cpu than copying from disk to user space.
For me, most indicative of performance issue / potential improvement is the
user / kernel cpu : Over all, it is 70% user cpu and 25% kernel. For a job that
essentially copies data from process to another, user cpu seem excessive
compared to kernel cpu. Extra buffer copies (see HADOOP-1702) are partly to
blame.
> Data node process consumes 180% cpu
> ------------------------------------
>
> Key: HADOOP-2144
> URL: https://issues.apache.org/jira/browse/HADOOP-2144
> Project: Hadoop
> Issue Type: Improvement
> Components: dfs
> Reporter: Runping Qi
>
> I did a test on DFS read throughput and found that the data node
> process consumes up to 180% cpu when it is under heavi load. Here are the
> details:
> The cluster has 380+ machines, each with 3GB mem and 4 cpus and 4 disks.
> I copied a 10GB file to dfs from one machine with a data node running there.
> Based on the dfs block placement policy, that machine has one replica for
> each block of the file.
> then I run 4 of the following commands in parellel:
> hadoop dfs -cat thefile > /dev/null &
> Since all the blocks have a local replica, all the read requests went to the
> local data node.
> I observed that:
> The data node process's cpu usage was around 180% for most of the time .
> The clients's cpu usage was moderate (as it should be).
> All the four disks were working concurrently with comparable read
> throughput.
> The total read throughput was maxed at 90MB/Sec, about 60% of the
> expected total
> aggregated max read throughput of 4 disks (160MB/Sec). Thus disks were
> not a bottleneck
> in this case.
> The data node's cpu usage seems unreasonably high.
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