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

[Zhou Wenjian]

Add descriptions of parallel dumping and how to use it.

Signed-off-by: Zhou Wenjian <zhouwj-fnst(a)cn.fujitsu.com&gt;
---
 kexec-kdump-howto.txt | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/kexec-kdump-howto.txt b/kexec-kdump-howto.txt
index 05b497f..97a319c 100644
--- a/kexec-kdump-howto.txt
+++ b/kexec-kdump-howto.txt
@@ -616,6 +616,38 @@ 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 substantially, especially in executing compress and filter.
+For example:
+
+	1."makedumpfile -c --num-threads [THREAD_NUM] /proc/vmcore dumpfile"
+	2."makedumpfile -c /proc/vmcore dumpfile",
+
+	1 has better performance than 2, if THREAD_NUM is larger than two
+	and the usable cpus number is larger than THREAD_NUM.
+
+Notes on how to use multiple cpus on a capture kernel on x86 system:
+
+Make sure 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.
+
+You need to specify how many cpus to be used 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.
+
+You should use necessary and suffcient amount of cpus on a capture kernel.
+IOW, don't use too many cpus on a capture kernel, or the capture kernel may
+lead to panic due to Out Of Memory(each cpu uses about 1MB).
+
+(*) 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