[PATCH v2 0/5] makedumpfile: --split: assign fair I/O workloads in appropriate time

[HATAYAMA Daisuke]

From: qiaonuohan <qiaonuohan at cn.fujitsu.com>
Subject: Re: [PATCH v2 0/5] makedumpfile: --split: assign fair I/O workloads in appropriate time
Date: Tue, 28 Oct 2014 14:32:12 +0800

> On 10/28/2014 02:24 PM, HATAYAMA Daisuke wrote:
>> From: Atsushi Kumagai<kumagai-atsushi at mxc.nes.nec.co.jp>
>> Subject: RE: [PATCH v2 0/5] makedumpfile: --split: assign fair I/O
>> workloads in appropriate time
>> Date: Mon, 27 Oct 2014 07:51:56 +0000
>>
>>> Hello Zhou,
>>>
>>>> On 10/17/2014 11:50 AM, Atsushi Kumagai wrote:
>>>>> Hello,
>>>>>
>>>>> The code looks good to me, thanks Zhou.
>>>>> Now, I have a question on performance.
>>>>>
>>>>>> The issue is discussed at
>>>>>> http://lists.infradead.org/pipermail/kexec/2014-March/011289.html
>>>>>>
>>>>>> This patch implements the idea of 2-pass algorhythm with smaller
>>>>>> memory to manage splitblock table.
>>>>>> Exactly the algorhythm is still 3-pass,but the time of second pass is
>>>>>> much shorter.
>>>>>> The tables below show the performence with different size of
>>>>>> cyclic-buffer and splitblock.
>>>>>> The test is executed on the machine having 128G memory.
>>>>>>
>>>>>> the value is total time (including first pass and second pass).
>>>>>> the value in brackets is the time of second pass.
>>>>>
>>>>> Do you have any idea why the time of second pass is much larger when
>>>>> the splitblock-size is 2G ? I worry about the scalability.
>>>>>
>>>> Hello,
>>>>
>>>> 	Since the previous machine can't be used for some reasons,I test several
>>>> 	times using latest code
>>>> in others, but that never happened. It seems that all things are
>>>> right. Tests are executed in two machines(server,pc).
>>>> Tests are based on:
>>>
>>> Well...OK, I'll take that as an issue specific to that machine
>>> (or your mistakes as you said).
>>> Now I have another question.
>>>
>>> calculate_end_pfn_by_splitblock():
>>> 	...
>>>          /* deal with incomplete splitblock */
>>>          if (pfn_needed_by_per_dumpfile<  0) {
>>>                  --*current_splitblock;
>>>                  splitblock_inner -= splitblock->entry_size;
>>>                  end_pfn = CURRENT_SPLITBLOCK_PFN_NUM;
>>>                  *current_splitblock_pfns = (-1) * pfn_needed_by_per_dumpfile;
>>>                  pfn_needed_by_per_dumpfile +=
>>>                  read_value_from_splitblock_table(splitblock_inner);
>>>                  end_pfn = calculate_end_pfn_in_cycle(CURRENT_SPLITBLOCK_PFN_NUM,
>>>                                                       CURRENT_SPLITBLOCK_PFN_NUM + splitblock->page_per_splitblock,
>>>                                                       end_pfn,pfn_needed_by_per_dumpfile);
>>>          }
>>>
>>> This block causes the re-scanning for the cycle corresponding to the
>>> current_splitblock, so the larger cyc-buf, the longer the time takes.
>>> If cyc-buf is 4096 (this means the number of cycle is 1), the whole
>>> page
>>> scanning will be done in the second pass. Actually, the performance
>>> when
>>> cyc-buf=4096 was so bad.
>>>
>>> Is this process necessary ? I think splitting splitblocks is overkill
>>> because I understood that splblk-size is the granularity of the
>>> fairness I/O, tuning splblk-size is a trade off between fairness and
>>> memory usage.
>>> However, there is no advantage to reducing splblk-size in the current
>>> implementation, it just consumes large amounts of memory.
>>> If we remove the process, we can avoid the whole page scanning in
>>> the second pass and reducing splblk-size will be meaningful as I
>>> expected.
>>>
>>
>> Yes, I don't think this rescan works with this splitblock method,
>> too. The idea of this splitblock method is to reduce the number of
>> filitering processing from 3-times to 2-times at the expence of at
>> most splitblock-size difference of each dump file. Doing rescan here
>> doesn't fit to the idea.
> 
> Hello,
> 
> The only things that bothers me is without getting the exact pfn, some
> of the split files may be empty, with no page stored in it. If this is
> not a issue, I think the re-scanning is useless.
> 

It is within the idea I wrote above that empty files can occur. But
there might be further improvement point to decrease possibility of
empty files. For example, how about deriving default splitblock size
from the actual number of dumpable pages, not constant 1GB?

--
Thanks.
HATAYAMA, Daisuke