[
https://issues.apache.org/jira/browse/HADOOP-9601?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13757097#comment-13757097
]
Trevor Robinson commented on HADOOP-9601:
-----------------------------------------
While unaligned loads should be avoided whenever possible for portability and
performance, ARMv6 and later (which includes low-end ARM11 implementations such
as Raspberry Pi) do [support unaligned word and half-word accesses in most
cases|http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0301h/Cdfejcbh.html]
by default. In other cases, the [Linux kernel will trap and fix the
access|http://lxr.free-electrons.com/source/arch/arm/mm/alignment.c?a=arm] by
default (with the obvious performance penalty). The specifics are complicated
([no
atomicity|http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0301h/Cdffhdje.html],
[no kernel support for unaligned
floats|http://jsolano.net/2012/09/06/arm-unaligned-data-access-and-floating-point-in-linux/],
etc.), but the point is that this fix is likely to benefit ARM even if it must
initially do unaligned 32-bit loads. Linux also provides unaligned access
support on other architectures, such as PowerPC, though the overhead may be
higher.
> Support native CRC on byte arrays
> ---------------------------------
>
> Key: HADOOP-9601
> URL: https://issues.apache.org/jira/browse/HADOOP-9601
> Project: Hadoop Common
> Issue Type: Improvement
> Components: performance, util
> Affects Versions: 3.0.0
> Reporter: Todd Lipcon
> Assignee: Gopal V
> Labels: perfomance
> Attachments: HADOOP-9601-bench.patch,
> HADOOP-9601-rebase+benchmark.patch, HADOOP-9601-trunk-rebase-2.patch,
> HADOOP-9601-trunk-rebase.patch, HADOOP-9601-WIP-01.patch,
> HADOOP-9601-WIP-02.patch
>
>
> When we first implemented the Native CRC code, we only did so for direct byte
> buffers, because these correspond directly to native heap memory and thus
> make it easy to access via JNI. We'd generally assumed that accessing byte[]
> arrays from JNI was not efficient enough, but now that I know more about JNI
> I don't think that's true -- we just need to make sure that the critical
> sections where we lock the buffers are short.
--
This message is automatically generated by JIRA.
If you think it was sent incorrectly, please contact your JIRA administrators
For more information on JIRA, see: http://www.atlassian.com/software/jira
