Atsushi,
Please see response below!
eric
On 07/11/2017 02:43 AM, Atsushi Kumagai wrote:
Hello Eric,
On 07/07/2017 04:09 AM, Atsushi Kumagai wrote:
The PFN_EXCLUDED value is used to control at which point a run of
zeros in the bitmap (zeros denote excluded pages) is large enough
to warrant truncating the current output segment and to create a
new output segment (containing non-excluded pages), in an ELF dump.
If the run is smaller than PFN_EXCLUDED, then those excluded pages
are still output in the ELF dump, for the current output segment.
By using smaller values of PFN_EXCLUDED, the resulting dump file
size can be made smaller by actually removing more excluded pages
from the resulting dump file.
This patch adds the command line option --exclude-threshold=<value>
to indicate the threshold. The default is 256, the legacy value
of PFN_EXCLUDED. The smallest value permitted is 1.
Using an existing vmcore, this was tested by the following:
% makedumpfile -E -d31 --exclude-threshold=256 -x vmlinux vmcore
newvmcore256
% makedumpfile -E -d31 --exclude-threshold=4 -x vmlinux vmcore
newvmcore4
I utilize -d31 in order to exclude as many page types as possible,
resulting in a [significantly] smaller file sizes than the original
vmcore.
-rwxrwx--- 1 edevolde edevolde 4034564096 Jun 27 10:24 vmcore
-rw------- 1 edevolde edevolde 119808156 Jul 6 13:01 newvmcore256
-rw------- 1 edevolde edevolde 100811276 Jul 6 13:08 newvmcore4
The use of smaller value of PFN_EXCLUDED increases the number of
output segments (the 'Number of program headers' in the readelf
output) in the ELF dump file.
How will you tune the value ? I'm not sure what is the benefit of the
tunable PFN_EXCLUDED. If there is no regression caused by too many
PT_LOAD
entries, I think we can decide a concrete PFN_EXCLUDED.
Allow me note two things prior to addressing the question.
Note that the value for PFN_EXCLUDED really should be in the range:
1 <= PFN_EXCLUDED <= NUM_PAGES(largest segment)
but that values larger than NUM_PAGES(largest segment) behave the same
as NUM_PAGES(largest segment) and simply prevent makedumpfile from ever
omitting excluded pages from the dump file.
Also note that the ELF header allows for a 16-bit e_phnum value for the
number of segments in the dump file. As it stands today, I doubt that
anybody has come close to reaching 65535 segments, but the combination
of larger and larger memories as well as the work we (Oracle) are doing
to further enhance the capabilities of makedumpfile, I believe we will
start to challenge this 65535 number.
I overlooked the limitation of the number of segments, so I considered
only "The first benefit" you said below.
The ability to tune PFN_EXCLUDED allows one to minimize file size while
still staying within ELF boundaries.
There are two ways in which have PFN_EXCLUDED as a tunable parameter
benefits the user.
The first benefit is, when making PFN_EXCLUDED smaller, makedumpfile has
more opportunities to NOT write excluded pages to the resulting dump
file, thus obtaining a smaller overall dump file size. And since a
PT_LOAD header is smaller than a page, this penalty (of more segments)
will always result in a smaller file size. (In the example I cite the
dump file was 18MB smaller with a PFN_EXCLUDED value of 4 than default
256, in spite of increasing the number of segments from 6 to 244).
The second benefit is, when enabling PFN_EXCLUDED to become larger, it
allows makedumpfile to continue to generate valid ELF dump files in the
presence of larger and larger memory systems. Generally speaking, the
goal is to minimize the size of dump files via the exclusion of
uninteresting pages (ie zero, free, etc), especially as the size of
memory continues to grow and grow. As the memory increases, there are
more and more of these uninteresting pages, and more opportunities for
makedumpfile to omit them (even at the current PFN_EXCLUDED value of
256). Furthermore, we are working on additional page exclusion
strategies that will drive more and more opportunities for makedumpfile
to omit these pages from the dump file. And as makedumpfile omits more
and more pages from the dump file, that increases the number of segments
needed.
By enabling a user to tune the value of PFN_EXCLUDED, we provide an
additional mechanism to balance the size of the ELF dump file with
respect to the size of memory.
It occurred to me that offering the option "--exclude-threshold=auto"
whereby a binary search on the second bitmap in the function
get_loads_dumpfile_cyclic() to determine the optimum value of
PFN_EXCLUDED (optimum here meaning the smallest possible value while
still staying within 65535 segments, which would yield the smallest
possible dump file size for the given constraints) would be an excellent
feature to have?
I think the "auto" is necessary for --exclude-threshold, the optimum
value should be calculated automatically. Otherwise, it imposes trial-and-error
on users every time, it doesn't sound practical. IOW, this patch is
unacceptable if there is no mechanism to support users.
So now, my only concern for this option is the processing time of the
binary search.
OK, so the idea of "tuning" the value of PFN_EXCLUDED is agree-able,
great! I will work on the binary search and report back with
measurements on the processing time of 'crash'. From there we can
determine if benefit is worthwhile.
Regards,
eric
[snip]
And with a larger number of segments, loading both vmcore and newvmcore4
into 'crash' resulted in identical outputs when run with the dmesg, ps,
files, mount, and net sub-commands.
What about the processing speed of crash, is there no slow down ?
I did not observe a noticeable change in processing speed of crash.
Good, it would be better to be represented by actual measured results.
Thanks,
Atsushi Kumagai
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