Add descriptions of parallel dumping and how to use it.
Signed-off-by: Zhou wenjian <zhouwj-fnst(a)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
@@ -616,6 +616,50 @@ options are copied from /proc/cmdline. In general it is best to
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.
+"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
+- 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
+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
+(*) 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.
- One can drop into a shell before/after saving vmcore with the help of