[PATCH] kexec-kdump-howto:Add introduction of parallel dumping

[Minfei Huang]

Hi, Wenjian.

How about setting the nr_cpus as default value for parallel dumping?
Then we do not need to specify the nr_cpus in kdump.conf. 

Thanks
Minfei

On 09/08/15 at 10:22am, Zhou Wenjian wrote:
> Add descriptions of parallel dumping and how to use it.
> 
> Signed-off-by: Zhou wenjian <zhouwj-fnst at cn.fujitsu.com>
> Signed-off-by: HATAYAMA Daisuke <d.hatayama at jp.fujitsu.com>
> ---
>  kexec-kdump-howto.txt | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 44 insertions(+)
> 
> diff --git a/kexec-kdump-howto.txt b/kexec-kdump-howto.txt
> index 05b497f..fab7b09 100644
> --- a/kexec-kdump-howto.txt
> +++ b/kexec-kdump-howto.txt
> @@ -616,6 +616,50 @@ options are copied from /proc/cmdline. In general it is best to append
>  command line options using "KDUMP_COMMANDLINE_APPEND=" instead of replacing
>  the original command line completely.
>  
> +Parallel Dumping Operation
> +==========================
> +Kexec allows kdump using multiple cpus. So parallel feature can
> +accelerate dumping greatly, especially in doing compress and filter.
> +For example:
> +"makedumpfile -c --num-threads [THREAD_NUM] /proc/vmcore dumpfile"
> +has 2 or more times performance of  
> +"makedumpfile -c /proc/vmcore dumpfile",
> +if THREAD_NUM is larger than 2 and the num of cpus that can be used is
> +larger than THREAD_NUM.
> +
> +Notes on how to use multiple cpus on a capture kernel on x86 system:
> +
> +To use multiple cpus on a capture kernel on x86 system:
> +
> +- First, confirm that you are using a sufficiently new kernel version
> +  that supports disable_cpu_apicid kernel option as a capture kernel,
> +  which is needed to avoid x86 specific hardware issue (*). The
> +  disable_cpu_apicid kernel option is automatically appended by
> +  kdumpctl script and is ignored if the kernel doesn't support
> +  it. Thus, you don't need to do anything else except for the
> +  confirmation.
> +
> +- Then, you need to specify how many cpus you use in a capture kernel
> +  by specifying the number of cpus in nr_cpus kernel option in
> +  /etc/sysconfig/kdump. nr_cpus is 1 at default.
> +
> +Note strongly that you should use necessary and sufficnet amount of
> +cpus on a capture kernel. IOW, don't use too many cpus on a capture
> +kernel, or the capture kernel easily leads to panic due to Out Of
> +Memory.
> +
> +There are kernel data structures and drivers allocating memory in
> +proportion to the number of cpus. More you use cpus, more and more
> +memory system consumes. Memory is rare, limited resource in a capture
> +kernel. Reserved memory should be kept as less as possible. When
> +configuring nr_cpus option, you should confirm that kdump certainly
> +successfully works without leading to panic due to Out Of Memory on a
> +capture kernel.
> +
> +(*) Without disable_cpu_apicid kernel option, capture kernel leads to
> +hang, system reset or power-off at boot, depending on your system and
> +runtime situation at the time of crash.
> +
>  Debugging Tips
>  --------------
>  - One can drop into a shell before/after saving vmcore with the help of
> -- 
> 1.8.3.1
> 
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