Message-Id: <20130215125701.75d134af8fd3d99f7853e014@mxc.nes.nec.co.jp>
Date:	Fri, 15 Feb 2013 12:57:01 +0900
From:	Atsushi Kumagai <kumagai-atsushi@....nes.nec.co.jp>
To:	d.hatayama@...fujitsu.com
Cc:	ebiederm@...ssion.com, vgoyal@...hat.com, cpw@....com,
	lisa.mitchell@...com, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [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@...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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