linux-kernel - Re: [PATCH 0/3] Early use of boot service memory

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Date:	Fri, 15 Nov 2013 14:24:25 -0800
From:	Yinghai Lu <yinghai@...nel.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	jerry.hoemann@...com, Pekka Enberg <penberg@...nel.org>,
	Rob Landley <rob@...dley.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	x86 maintainers <x86@...nel.org>,
	Matt Fleming <matt.fleming@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"list@...ederm.org:DOCUMENTATION" <linux-doc@...r.kernel.org>,
	"list@...ederm.org:MEMORY MANAGEMENT" <linux-mm@...ck.org>,
	linux-efi@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Early use of boot service memory

On Fri, Nov 15, 2013 at 10:03 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Fri, Nov 15, 2013 at 09:33:41AM -0800, Yinghai Lu wrote:

>> I have one system with 6TiB memory, kdump does not work even
>> crashkernel=512M in legacy mode. ( it only work on system with
>> 4.5TiB).
>
> Recently I tested one system with 6TB of memory and dumped successfully
> with 512MB reserved under 896MB. Also I have heard reports of successful
> dump of 12TB system with 512MB reserved below 896MB (due to cyclic
> mode of makedumpfile).
>
> So with newer releases only reason one might want to reserve more
> memory is that it might provide speed benefits. We need more testing
> to quantify this.

You may need bunch of PCIe cards installed.

The system with 6TiB + 16 PCIe cards, second kernel OOM.
The system with 4.5TiB + 16 PCIe cards, second kernel works with vmcore dumped.

>
>> --- first kernel can reserve the 512M under 896M, second kernel will
>> OOM as it load driver for every pci devices...
>>
>> So why would RH guys not spend some time on optimizing your kdump initrd
>> build scripts and only put dump device related driver in it?
>
> Try latest Fedora and that's what we do. Now we have moved to dracut
> based initramfs generation and we tell dracut that build initramfs for
> host and additional dump destination and dracut builds it for those only.
> I think there might be scope for further optimization, but I don't think
> that's the problem any more.

Good. Assume that will be in RHEL 7.

>
> So issue remains that crashkernel=X,high is not a good default choice
> because it consumes extra 72M which we don't have to.

then if it falls into 896~4G, user may still need to update kexec-tools ?

Thanks

Yinghai
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