Re: [Xen-ia64-devel] [PATCH] Use saner dom0 memory and vcpu defaults, don't panic on over-allocation

[Alex Williamson <alex.williamson@xxxxxx>]

Hi Jarod,

On Sat, 2007-08-04 at 22:05 -0400, Jarod Wilson wrote:
> After poking at the x86 code again, it looks like there's simply a guess 
> there too for extra memory to reserve. From xen/arch/x86/domain_build.c:
> 
>      /*
>       * If domain 0 allocation isn't specified, reserve 1/16th of available
>       * memory for things like DMA buffers. This reservation is clamped to
>       * a maximum of 128MB.
>       */
>      if ( dom0_nrpages == 0 )
>      {
>          dom0_nrpages = avail;
>          dom0_nrpages = min(dom0_nrpages / 16, 128L << (20 - PAGE_SHIFT));
>          dom0_nrpages = -dom0_nrpages;
>      }
> 
> 
> Based on empirical testing, it seems something along the lines of 1MB 
> per 4GB of RAM, capped at like 64MB (though probably not necessary), 
> might be sane. Its not exact of course, since memory amount doesn't 
> necessarily indicate how many PCI root bridges there are to allocate, 
> etc., but it seems to be a reasonable guess... The attached is a crack 
> at that, and on your 96G machine, should decrease the default "all" 
> memory allocation by about 24MB, allowing it to fully boot.

   I think we're going to have to go with something like this, but why
would we reduce the cap to 64MB?  I usually think of ia64 systems as
having "bigger" I/O than x86, so it seems like maybe we want to stick
with at least 128MB(?)

> Here's another thought: we could play it extra-safe, and slide that 
> allocation down another 12-24MB, and possibly even switch to allocating 
> "all" memory to dom0 by default... Then allocate all cpus too, and we'd 
> have parity with x86 here... :)

   I'm not sure I see the value in that yet.  If you have a
32/64/128/etc-way system, do you really want to have all those extra
vCPUs floating around?  Don't forget, XenSource is targeting 4-socket
systems.  In that arena, it might make sense to allocate all CPUs, but
as you scale the system up, I'm not sure it continues to make sense.
Thanks,

        Alex

-- 
Alex Williamson                             HP Open Source & Linux Org.


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