[PATCH 0/2] makedumpfile: for large memories

HATAYAMA Daisuke d.hatayama at jp.fujitsu.com
Tue Jan 7 05:14:08 EST 2014

(2014/01/01 8:30), cpw wrote:
> From: Cliff Wickman <cpw at sgi.com>
> Gentlemen of kexec,
> I have been working on enabling kdump on some very large systems, and
> have found some solutions that I hope you will consider.
> The first issue is to work within the restricted size of crashkernel memory
> under 2.6.32-based kernels, such as sles11 and rhel6.
> The second issue is to reduce the very large size of a dump of a big memory
> system, even on an idle system.
> These are my propositions:
> Size of crashkernel memory
>    1) raw i/o for writing the dump
>    2) use root device for the bitmap file (not tmpfs)
>    3) raw i/o for reading/writing the bitmaps

Thanks for 1) and 3). I have the same idea of using direct i/o but have yet
to evaluate how it improves performance. This work is very helpful to me.

For 2), I understand the merit as long as non-cyclic mode is alive, but
there are issues we should consider. Root device could be broken in general
due to the bug that caused the crash, which means this reduces reliability
in some amount. At least this should be warned in help message. Also, you
need to deal with flattened format mode in which mode makedumpfile writes
dump data in standard output. I think it sufficient to disallow using this
new functionality together with -F option.

> Size of dump (and hence the duration of dumping)
>    4) exclude page structures for unused pages
> 1) Is quite easy.  The cache of pages needs to be aligned on a block
>    boundary and written in block multiples, as required by O_DIRECT files.
>    The use of raw i/o prevents the growing of the crash kernel's page
>    cache.
> 2) Is also quite easy.  My patch finds the path to the crash
>    kernel's root device by examining the dump pathname. Storing the bitmaps
>    to a file is otherwise not conserving memory, as they are being written
>    to tmpfs.
> 3) Raw i/o for the bitmaps, is accomplished by caching the
>    bitmap file in a similar way to that of the dump file.
>    I find that the use of direct i/o is not significantly slower than
>    writing through the kernel's page cache.
> 4) The excluding of unused kernel page structures is very
>    important for a large memory system.  The kernel otherwise includes
>    3.67 million pages of page structures per TB of memory. By contrast
>    the rest of the kernel is only about 1 million pages.
> Test results are below, for systems of 1TB, 2TB, 8.8TB and 16TB.
> (There are no 'old' numbers for 16TB as time and space requirements
>   made those effectively useless.)
> Run times were generally reduced 2-3x, and dump size reduced about 8x.
> All timings were done using 512M of crashkernel memory.
>     System memory size
>     1TB                     unpatched    patched
>       OS: rhel6.4 (does a free pages pass)
>       page scan time           1.6min    1.6min
>       dump copy time           2.4min     .4min
>       total time               4.1min    2.0min
>       dump size                 3014M      364M
>       OS: rhel6.5
>       page scan time            .6min     .6min
>       dump copy time           2.3min     .5min
>       total time               2.9min    1.1min
>       dump size                 3011M      423M
>       OS: sles11sp3 (3.0.93)
>       page scan time            .5min     .5min
>       dump copy time           2.3min     .5min
>       total time               2.8min    1.0min
>       dump size                 2950M      350M
>     2TB
>       OS: rhel6.5           (cyclicx3)
>       page scan time           2.0min    1.8min
>       dump copy time           8.0min    1.5min
>       total time              10.0min    3.3min
>       dump size                 6141M      835M
>     8.8TB
>       OS: rhel6.5           (cyclicx5)
>       page scan time           6.6min    5.5min
>       dump copy time          67.8min    6.2min
>       total time              74.4min   11.7min
>       dump size                 15.8G      2.7G
>     16TB
>       OS: rhel6.4
>       page scan time                   125.3min
>       dump copy time                    13.2min
>       total time                       138.5min
>       dump size                            4.0G
>       OS: rhel6.5
>       page scan time                    27.8min
>       dump copy time                    13.3min
>       total time                        41.1min
>       dump size                            4.1G

Could you tell me what kind of filesystem you use as dump partition?
Although I don't know filesystem things very much, I heard performance
of direct I/O depends much on filesystems.

Also, how did you measure these times? I've forgotten reporting this,
but surprisingly, I found reported time in cyclic-mode is different
from that in non-cyclic mode. In cyclic-mode, time for writing data
contains that for scanning pages. So, it must look larger than the

> Page scan time is greatly affected by whether or not the
> kernel supports mmap of /proc/vmcore.

Another idea of improving page scan time is to touch mmaped region
directly, not through readmem(), by which we can skip copying page
descriptors into buffers. Although I have yet to evaluate how much this
affects performance, it must amount to considerably large in total.

> The choice of snappy vs. zlib compression becomes fairly irrelevant
> when we can shrink the dump size dramatically.  The above
> were done with snappy compression.
> I am sending my 2 working patches.
> They are kludgy in the sense that they ignore all forms of
> kdump except the creation of a disk dump, and all architectures
> except x86_64.
> But I think they are sufficient to demonstrate the sizable
> time, crashkernel space and disk space savings that are possible.


More information about the kexec mailing list