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Date: Fri, 26 Jun 2015 15:27:41 +0800
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Subject: Re: [PATCH v2 00/10] makedumpfile: parallel processing
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To: Atsushi Kumagai <ats-kumagai@wm.jp.nec.com>
Cc: "kexec@lists.infradead.org" <kexec@lists.infradead.org>

On 06/26/2015 03:07 PM, Atsushi Kumagai wrote:
> Hello Zhou,
>
>> >Hello Atsushi Kumagai,
>> >
>> >I test this patch set in several machines and the following is the benchmark.
> Thanks for your report, it looks good as before.
> I also did simple test on kernel 3.19 on a 5GB Virtual Machine,
> but I can't get such good result as below:
>
> / # time makedumpfile -c --num-threads 0 /proc/vmcore /mnt/dumpfile
> Copying data                       : [100.0 %] /
>
> The dumpfile is saved to /mnt/dumpfile.
>
> makedumpfile Completed.
> real    0m 44.40s
> user    0m 43.62s
> sys     0m 0.69s
> / #
>
> / # time makedumpfile -c --num-threads 4 /proc/vmcore /mnt/dumpfile
> Copying data                       : [100.0 %] -
> Copying data                       : [100.0 %] /
>
> The dumpfile is saved to /mnt/dumpfile.
>
> makedumpfile Completed.
> real    5m 29.54s
> user    6m 8.18s
> sys     16m 33.25s
> / #
>
>
> There is a big performance degradation.
> Do you have any ideas why this happens ?

Does your Virtual Machine have more than 2 processors?
If so, check the value:nr_cpus in KDUMP_COMMANDLINE_APPEND of /etc/sysconfig/kdump.
It should be also set to the number larger than 2.

To "--num-threads 4", the most suitable number of processors is 5.

-- 
Thanks
Zhou Wenjian

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