[PATCH 0/2] makedumpfile: for large memories

Atsushi Kumagai kumagai-atsushi at mxc.nes.nec.co.jp
Mon Jan 6 04:27:34 EST 2014

Hello Cliff,

On 2014/01/01 8:30:47, kexec <kexec-bounces at lists.infradead.org> 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
> 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.

There is no reason to reject this idea, please re-post it as a formal patch.
If possible, I would like to know the benefit of only this.

> 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.

Users will expect that the size of dump file is the same as the size of
RAM at most, they will prepare a disk which fit to save that.
But 2) breaks this estimation, I worry about it a little.

Of course, I don't reject this idea just only for that reason,
but I would like to know the definite advantage of this.
I suppose that the improvement showed in your benchmarks may be came
from 1) and 4) mostly, so could you let me know that only 2) and 3)
can perform much faster than the current cyclic mode ?

> 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.

According to your and Dave's mails, 4) seems risky and unacceptable
for now. I think we need more investigation for this.

Atsushi Kumagai

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