[PATCH v2] makedumpfile: request the kernel do page scans

[HATAYAMA Daisuke]

From: Cliff Wickman <cpw at sgi.com>
Subject: Re: [PATCH v2] makedumpfile: request the kernel do page scans
Date: Mon, 7 Jan 2013 07:39:35 -0600

> An update on testing thise patches:
> 
>> This version of the patch improves the consolidation of the mem_map table
>> that is passed to the kernel.  See make_kernel_mmap().
>> Particularly the seemingly duplicate pfn ranges generated on an older
>> (2.6.32-based, rhel6) kernel.
> 
> 
> On a 2TB machine (idle) machine:
>   the crash kernel ran successfully in 512M
>   the scan for unnecessary pages takes about 40 seconds
>   it was a single scan, not cyclic
>   
> On a 16TB machine (idle) machine:
>   the crash kernel ran successfully in 512M
>   there is a pause of about 4 minutes during makedumpfile initialization
>   the scan for unnecessary pages takes about 4.8 minutes
>   it was a single scan, not cyclic
> 

I wonder why this didn't result in OOM on 16TB. Was that really done
in non-cyclic mode? Could you show me a log of makedumpfile? You can
create it by passing --message-level 31.

Also, to consider how to resolve this performance issue, this test is
not enough. We also need to benchmark that *without filtering in
kernel-space* but with making page tables, not using ioremap. As I
said before, I think the bad performance is caused by too many calls
of ioremap/iounmap. We then need to comapre two results of these
benchmarks.

Thanks.
HATAYAMA, Daisuke