[makedumpfile PATCH] Allow PFN_EXCLUDED to be tunable via command line option --exclude-threshold
Eric DeVolder
eric.devolder at oracle.com
Fri Jul 7 10:53:52 PDT 2017
Hi Atsushi,
please see below.
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.
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.
>
> The penalty for splitting PT_LOAD is the size of a PT_LOAD header,
> so the best PFN_EXCLUDED is the minimum number which meets the condition
> below:
>
> (size of PT_LOAD header) < (PFN_EXCLUDED << PAGE_SIZE)
>
I admit I don't quite understand, would you mind working through an
example or two?
It seems to me that a PT_LOAD header of 56 bytes is always less than a
page_size of 4096 bytes, and would always be a win (meaning a new
segment is better than dumping the page), especially as the consecutive
number of physically contiguous excluded pages is large.
The only caveat here being that we can not exceed e_phnum limitation of
65535 segments.
>> % readelf -h vmcore
>> ELF Header:
>> Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
>> Class: ELF64
>> Data: 2's complement, little endian
>> Version: 1 (current)
>> OS/ABI: UNIX - System V
>> ABI Version: 0
>> Type: CORE (Core file)
>> Machine: Advanced Micro Devices X86-64
>> Version: 0x1
>> Entry point address: 0x0
>> Start of program headers: 64 (bytes into file)
>> Start of section headers: 0 (bytes into file)
>> Flags: 0x0
>> Size of this header: 64 (bytes)
>> Size of program headers: 56 (bytes)
>> Number of program headers: 6
>> ^^^
>> Size of section headers: 0 (bytes)
>> Number of section headers: 0
>> Section header string table index: 0
>>
>> % readelf -h newvmcore256
>> ELF Header:
>> Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
>> Class: ELF64
>> Data: 2's complement, little endian
>> Version: 1 (current)
>> OS/ABI: UNIX - System V
>> ABI Version: 0
>> Type: CORE (Core file)
>> Machine: Advanced Micro Devices X86-64
>> Version: 0x1
>> Entry point address: 0x0
>> Start of program headers: 64 (bytes into file)
>> Start of section headers: 0 (bytes into file)
>> Flags: 0x0
>> Size of this header: 64 (bytes)
>> Size of program headers: 56 (bytes)
>> Number of program headers: 18
>> ^^^
>> Size of section headers: 0 (bytes)
>> Number of section headers: 0
>> Section header string table index: 0
>>
>> % readelf -h newvmcore4
>> ELF Header:
>> Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
>> Class: ELF64
>> Data: 2's complement, little endian
>> Version: 1 (current)
>> OS/ABI: UNIX - System V
>> ABI Version: 0
>> Type: CORE (Core file)
>> Machine: Advanced Micro Devices X86-64
>> Version: 0x1
>> Entry point address: 0x0
>> Start of program headers: 64 (bytes into file)
>> Start of section headers: 0 (bytes into file)
>> Flags: 0x0
>> Size of this header: 64 (bytes)
>> Size of program headers: 56 (bytes)
>> Number of program headers: 244
>> ^^^
>> Size of section headers: 0 (bytes)
>> Number of section headers: 0
>> Section header string table index: 0
>>
>> The newvmcore4 has an even smaller file size than newvmcore256, with
>> the small price being that there are now 244 rather than 18 segments
>> in the dump file.
>>
>> 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.
>
>
> Thanks,
> Atsushi Kumagai
>
>> Signed-off-by: Eric DeVolder <eric.devolder at oracle.com>
>> ---
>> v1: Posted 06jul2017 to kexec-tools mailing list
>> - original
>> ---
>> makedumpfile.c | 20 +++++++++++++++++---
>> makedumpfile.h | 4 +++-
>> 2 files changed, 20 insertions(+), 4 deletions(-)
>>
>> diff --git a/makedumpfile.c b/makedumpfile.c
>> index e69b6df..940f64c 100644
>> --- a/makedumpfile.c
>> +++ b/makedumpfile.c
>> @@ -7236,7 +7236,7 @@ get_loads_dumpfile_cyclic(void)
>>
>> /*
>> * If the number of the contiguous pages to be excluded
>> - * is 256 or more, those pages are excluded really.
>> + * is PFN_EXCLUDED or more, those pages are excluded really.
>> * And a new PT_LOAD segment is created.
>> */
>> if (num_excluded >= PFN_EXCLUDED) {
>> @@ -7352,7 +7352,7 @@ write_elf_pages_cyclic(struct cache_data *cd_header, struct cache_data *cd_page)
>> continue;
>> /*
>> * If the number of the contiguous pages to be excluded
>> - * is 255 or less, those pages are not excluded.
>> + * is less than PFN_EXCLUDED, those pages are not excluded.
>> */
>> } else if (num_excluded < PFN_EXCLUDED) {
>> if ((pfn == pfn_end - 1) && frac_tail) {
>> @@ -7370,7 +7370,7 @@ write_elf_pages_cyclic(struct cache_data *cd_header, struct cache_data *cd_page)
>>
>> /*
>> * If the number of the contiguous pages to be excluded
>> - * is 256 or more, those pages are excluded really.
>> + * is PFN_EXCLUDED or more, those pages are excluded really.
>> * And a new PT_LOAD segment is created.
>> */
>> load.p_memsz = memsz;
>> @@ -11007,6 +11007,7 @@ static struct option longopts[] = {
>> {"splitblock-size", required_argument, NULL, OPT_SPLITBLOCK_SIZE},
>> {"work-dir", required_argument, NULL, OPT_WORKING_DIR},
>> {"num-threads", required_argument, NULL, OPT_NUM_THREADS},
>> + {"exclude-threshold", required_argument, NULL, OPT_PFN_EXCLUDE_THRESHOLD},
>> {0, 0, 0, 0}
>> };
>>
>> @@ -11044,6 +11045,14 @@ main(int argc, char *argv[])
>> */
>> info->flag_usemmap = MMAP_TRY;
>>
>> + /*
>> + * A run of zeros in the bitmap (excluded pages) of less than
>> + * pfn_excluded_threshold in length will still be dumped. Runs greater
>> + * than or equal to pfn_excluded_threshold will result in the creation
>> + * of a new output segment, for ELF dumps.
>> + */
>> + info->pfn_exclude_threshold = 256;
>> +
>> info->block_order = DEFAULT_ORDER;
>> message_level = DEFAULT_MSG_LEVEL;
>> while ((opt = getopt_long(argc, argv, "b:cDd:eEFfg:hi:lpRvXx:", longopts,
>> @@ -11163,6 +11172,11 @@ main(int argc, char *argv[])
>> case OPT_NUM_THREADS:
>> info->num_threads = MAX(atoi(optarg), 0);
>> break;
>> + case OPT_PFN_EXCLUDE_THRESHOLD:
>> + info->pfn_exclude_threshold = strtoul(optarg, NULL, 0);
>> + if (0 == info->pfn_exclude_threshold)
>> + info->pfn_exclude_threshold = 1;
>> + break;
>> case '?':
>> MSG("Commandline parameter is invalid.\n");
>> MSG("Try `makedumpfile --help' for more information.\n");
>> diff --git a/makedumpfile.h b/makedumpfile.h
>> index e32e567..33d3eb0 100644
>> --- a/makedumpfile.h
>> +++ b/makedumpfile.h
>> @@ -216,7 +216,7 @@ isAnon(unsigned long mapping)
>>
>> #define BITPERBYTE (8)
>> #define PGMM_CACHED (512)
>> -#define PFN_EXCLUDED (256)
>> +#define PFN_EXCLUDED (info->pfn_exclude_threshold)
>> #define BUFSIZE (1024)
>> #define BUFSIZE_FGETS (1500)
>> #define BUFSIZE_BITMAP (4096)
>> @@ -1139,6 +1139,7 @@ struct DumpInfo {
>> long page_size; /* size of page */
>> long page_shift;
>> mdf_pfn_t max_mapnr; /* number of page descriptor */
>> + unsigned long pfn_exclude_threshold;
>> unsigned long page_offset;
>> unsigned long section_size_bits;
>> unsigned long max_physmem_bits;
>> @@ -2143,6 +2144,7 @@ struct elf_prstatus {
>> #define OPT_SPLITBLOCK_SIZE OPT_START+14
>> #define OPT_WORKING_DIR OPT_START+15
>> #define OPT_NUM_THREADS OPT_START+16
>> +#define OPT_PFN_EXCLUDE_THRESHOLD OPT_START+17
>>
>> /*
>> * Function Prototype.
>> --
>> 2.7.4
>
>
>
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