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SubjectRe: [PATCH 00/13] kdump, vmcore: support mmap() on /proc/vmcore
Hello HATAYAMA-san,

On Thu, 14 Feb 2013 19:11:43 +0900
HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com> wrote:

> Currently, read to /proc/vmcore is done by read_oldmem() that uses
> ioremap/iounmap per a single page. For example, if memory is 1GB,
> ioremap/iounmap is called (1GB / 4KB)-times, that is, 262144
> times. This causes big performance degradation.
>
> To address the issue, this patch implements mmap() on /proc/vmcore to
> improve read performance. My simple benchmark shows the improvement
> from 200 [MiB/sec] to over 50.0 [GiB/sec].

Thanks for your hard work, I think it's a good enough improvement.

> Benchmark
> =========
>
> = Machine spec
> - CPU: Intel(R) Xeon(R) CPU E7- 4820 @ 2.00GHz (4 sockets, 8 cores) (*)
> - memory: 32GB
> - kernel: 3.8-rc6 with this patch
> - vmcore size: 31.7GB
>
> (*) only 1 cpu is used in the 2nd kernel now.
>
> = Benchmark Case
>
> 1) copy /proc/vmcore *WITHOUT* mmap() on /proc/vmcore
>
> $ time dd bs=4096 if=/proc/vmcore of=/dev/null
> 8307246+1 records in
> 8307246+1 records out
> real 2m 31.50s
> user 0m 1.06s
> sys 2m 27.60s
>
> So performance is 214.26 [MiB/sec].
>
> 2) copy /proc/vmcore with mmap()
>
> I ran the next command and recorded real time:
>
> $ for n in $(seq 1 15) ; do \
> > time copyvmcore2 --blocksize=$((4096 * (1 << (n - 1)))) /proc/vmcore /dev/null \
> > done
>
> where copyvmcore2 is an ad-hoc test tool that read data from
> /proc/vmcore via mmap() in given block-size unit and write them to
> some file.
>
> | n | map size | time | page table | performance |
> | | | (sec) | | [GiB/sec] |
> |----+----------+-------+------------+-------------|
> | 1 | 4 KiB | 78.35 | 8 iB | 0.40 |
> | 2 | 8 KiB | 45.29 | 16 iB | 0.70 |
> | 3 | 16 KiB | 23.82 | 32 iB | 1.33 |
> | 4 | 32 KiB | 12.90 | 64 iB | 2.46 |
> | 5 | 64 KiB | 6.13 | 128 iB | 5.17 |
> | 6 | 128 KiB | 3.26 | 256 iB | 9.72 |
> | 7 | 256 KiB | 1.86 | 512 iB | 17.04 |
> | 8 | 512 KiB | 1.13 | 1 KiB | 28.04 |
> | 9 | 1 MiB | 0.77 | 2 KiB | 41.16 |
> | 10 | 2 MiB | 0.58 | 4 KiB | 54.64 |
> | 11 | 4 MiB | 0.50 | 8 KiB | 63.38 |
> | 12 | 8 MiB | 0.46 | 16 KiB | 68.89 |
> | 13 | 16 MiB | 0.44 | 32 KiB | 72.02 |
> | 14 | 32 MiB | 0.44 | 64 KiB | 72.02 |
> | 15 | 64 MiB | 0.45 | 128 KiB | 70.42 |
>
> 3) copy /proc/vmcore with mmap() on /dev/oldmem
>
> I posted another patch series for mmap() on /dev/oldmem a few weeks ago.
> See: https://lkml.org/lkml/2013/2/3/431
>
> Next is the table shown on the post showing the benchmark.
>
> | n | map size | time | page table | performance |
> | | | (sec) | | [GiB/sec] |
> |----+----------+-------+------------+-------------|
> | 1 | 4 KiB | 41.86 | 8 iB | 0.76 |
> | 2 | 8 KiB | 25.43 | 16 iB | 1.25 |
> | 3 | 16 KiB | 13.28 | 32 iB | 2.39 |
> | 4 | 32 KiB | 7.20 | 64 iB | 4.40 |
> | 5 | 64 KiB | 3.45 | 128 iB | 9.19 |
> | 6 | 128 KiB | 1.82 | 256 iB | 17.42 |
> | 7 | 256 KiB | 1.03 | 512 iB | 30.78 |
> | 8 | 512 KiB | 0.61 | 1K iB | 51.97 |
> | 9 | 1 MiB | 0.41 | 2K iB | 77.32 |
> | 10 | 2 MiB | 0.32 | 4K iB | 99.06 |
> | 11 | 4 MiB | 0.27 | 8K iB | 117.41 |
> | 12 | 8 MiB | 0.24 | 16 KiB | 132.08 |
> | 13 | 16 MiB | 0.23 | 32 KiB | 137.83 |
> | 14 | 32 MiB | 0.22 | 64 KiB | 144.09 |
> | 15 | 64 MiB | 0.22 | 128 KiB | 144.09 |
>
> = Discussion
>
> - For small map size, we can see performance degradation on mmap()
> case due to many page table modification and TLB flushes similarly
> to read_oldmem() case. But for large map size we can see the
> improved performance.
>
> Each application need to choose appropreate map size for their
> preferable performance.
>
> - mmap() on /dev/oldmem appears better than that on /proc/vmcore. But
> actual processing does not only copying but also IO work. This
> difference is not a problem.

To keep the makedumpfile code simple, I wouldn't like to use /dev/oldmem
as another input interface. And I hope that we can get enough performance
with only /proc/vmcore.

> - Both mmap() case shows drastically better performance than previous
> RFC patch set's about 2.5 [GiB/sec] that maps all dump target memory
> in kernel direct mapping address space. This is because there's no
> longer memcpy() from kernel-space to user-space.
>
> Design
> ======
>
> = Support Range
>
> - mmap() on /proc/vmcore is supported on ELF64 interface only. ELF32
> interface is used only if dump target size is less than 4GB. Then,
> the existing interface is enough in performance.
>
> = Change of /proc/vmcore format
>
> For mmap()'s page-size boundary requirement, /proc/vmcore changed its
> own shape and now put its objects in page-size boundary.
>
> - Allocate buffer for ELF headers in page-size boundary.
> => See [PATCH 01/13].
>
> - Note objects scattered on old memory are copied in a single
> page-size aligned buffer on 2nd kernel, and it is remapped to
> user-space.
> => See [PATCH 09/13].
>
> - The head and/or tail pages of memroy chunks are also copied on 2nd
> kernel if either of their ends is not page-size aligned. See
> => See [PATCH 12/13].
>
> = 32-bit PAE limitation
>
> - On 32-bit PAE limitation, mmap_vmcore() can handle upto 16TB memory
> only since remap_pfn_range()'s third argument, pfn, has 32-bit
> length only, defined as unsigned long type.
>
> TODO
> ====
>
> - fix makedumpfile to use mmap() on /proc/vmcore and benchmark it to
> confirm whether we can see enough performance improvement.

As a first step, I'll make a prototype patch for benchmarking unless you
have already done it.


Thanks
Atsushi Kumagai

>
> Test
> ====
>
> Done on x86-64, x86-32 both with 1GB and over 4GB memory environments.
>
> ---
>
> HATAYAMA Daisuke (13):
> vmcore: introduce mmap_vmcore()
> vmcore: copy non page-size aligned head and tail pages in 2nd kernel
> vmcore: count holes generated by round-up operation for vmcore size
> vmcore: round-up offset of vmcore object in page-size boundary
> vmcore: copy ELF note segments in buffer on 2nd kernel
> vmcore: remove unused helper function
> vmcore: modify read_vmcore() to read buffer on 2nd kernel
> vmcore: modify vmcore clean-up function to free buffer on 2nd kernel
> vmcore: modify ELF32 code according to new type
> vmcore: introduce types for objects copied in 2nd kernel
> vmcore: fill unused part of buffer for ELF headers with 0
> vmcore: round up buffer size of ELF headers by PAGE_SIZE
> vmcore: allocate buffer for ELF headers on page-size alignment
>
>
> fs/proc/vmcore.c | 408 +++++++++++++++++++++++++++++++++++------------
> include/linux/proc_fs.h | 11 +
> 2 files changed, 313 insertions(+), 106 deletions(-)
>
> --
>
> Thanks.
> HATAYAMA, Daisuke


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