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

[Vivek Goyal <vgoyal@redhat.com>]

On Fri, Nov 15, 2013 at 09:33:41AM -0800, Yinghai Lu wrote:

[..]
> > I think crashkernel=X,high is not a good default choice for distros.
> > Reserving memory high reserves 72MB (or more) low memory for swiotlb. We
> > work hard to keep crashkernel memory amount low and currently reserve
> > 128M by default. Now suddenly our total memory reservation will shoot
> > to 200 MB if we choose ,high option. That's jump of more than 50%. It
> > is not needed.
> 
> If the system support intel IOMMU, we only need to that 72M for SWIOTLB
> or AMD workaround.
> If the user really care that for intel iommu enable system, they could use
> "crashkernel=0,low" to have that 72M back.
> 
> and that 72M is under 4G instead of 896M.
> 
> so reserve 72M is not better than reserve 128M?

This 72M is on top of 128M reserved. Also IOMMU support is very flaky
with kdump and in fact on most of the system it might not work. So
majority of systems will pay this cost of 72M.

> 
> >
> > We can do dumping operation successfully in *less* reserved memory by
> > reserving memory below 4G. And hence crashkernel=,high is not a good
> > default.
> >
> > Instead, crashkernel=X is a good default if we are ready to change
> > semantics a bit. If sufficient crashkernel memory is not available
> > in low memory area, look for it above 4G. This incurs penalty of
> > 72M *only* if it has to and not by default on most of the systems.
> >
> > And this should solve jerry's problem too on *latest* kernels. For
> > older kernels, we don't have ,high support. So using that is not
> > an option. (until and unless somebody is ready to backport everything
> > needed to boot old kernel above 4G).
> 
> that problem looks not related.
> 
> 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.

> --- 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. 

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

Thanks
Vivek

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