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https://issues.apache.org/jira/browse/HADOOP-1702?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12575553#action_12575553
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Raghu Angadi commented on HADOOP-1702:
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Test results show *30%* improvement in DataNode CPU with the patch. I think it
makes sense. Based on the picture above before this patch, with replication of
3, the data is copied 6 + 6 + 4 times and with this patch it is 3 + 3 + 2. Each
of these datanodes verify CRC. Approximating cost of checksumming to be twice
that of a memory copy, we get (8+6)/(14+6) == 70%. If we increase the size of
checksum chunk, cost of CRC will go down. It will be 68% with a factor of 1.5
for CRC.
Test Setup : three instances of 'dd if=/dev/zero 4Gb | hadoop -put - 4Gb'. More
importantly, DataNode was modified to write the deta to '/dev/null' instead of
the block file. Otherwise I could not isolate the test from disk activity. The
cluster has 3 datanodes. The clients, Namenode, and datanodes are all running
on the same node. The test was CPU bound.
CPU measurement : Linux reports a process' cpu in /proc/pid/stat : 14th entry
is user cpu and 15th is kernel cpu. I think these are specified in jiffies.
Like most things with Linux kernel, these are approximations but reasonably
dependable in large numbers.
below: 'u' and 'k' are user and kernel cpu in thousands of jiffies.
|| Test || Run 1 || Run 2 || Run 3 || Avg Total Cpu || Avg Time ||
| Trunk* | 8.60u 2.52k 372s | 8.36u 2.48k 368s | 8.39u 2.40k 368s | *10.95* |
369s |
| Trunk + patch* | 5.61u 2.22k 289s | 5.38u 2.16k 296s | 5.57u 2.25k 289s |
*7.73 (70%)*| 291s (79%) |
{{*}} : datanodes write data to /dev/null.
Currently, DFSIO benchmark shows dip in write b/w. I am still looking into it.
> Reduce buffer copies when data is written to DFS
> ------------------------------------------------
>
> Key: HADOOP-1702
> URL: https://issues.apache.org/jira/browse/HADOOP-1702
> Project: Hadoop Core
> Issue Type: Bug
> Components: dfs
> Affects Versions: 0.14.0
> Reporter: Raghu Angadi
> Assignee: Raghu Angadi
> Fix For: 0.17.0
>
> Attachments: HADOOP-1702.patch
>
>
> HADOOP-1649 adds extra buffering to improve write performance. The following
> diagram shows buffers as pointed by (numbers). Each eatra buffer adds an
> extra copy since most of our read()/write()s match the io.bytes.per.checksum,
> which is much smaller than buffer size.
> {noformat}
> (1) (2) (3) (5)
> +---||----[ CLIENT ]---||----<>-----||---[ DATANODE ]---||--<>-> to Mirror
>
> | (buffer) (socket) | (4)
> | +--||--+
> ===== |
> ===== =====
> (disk) =====
> {noformat}
> Currently loops that read and write block data, handle one checksum chunk at
> a time. By reading multiple chunks at a time, we can remove buffers (1), (2),
> (3), and (5).
> Similarly some copies can be reduced when clients read data from the DFS.
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