Re: [PATCH] x86/pod: Do not fragment PoD memory allocations

[Elliott Mitchell <ehem+xen@xxxxxxx>]

On Thu, Jan 28, 2021 at 10:42:27PM +0000, George Dunlap wrote:
> 
> > On Jan 28, 2021, at 6:26 PM, Elliott Mitchell <ehem+xen@xxxxxxx> wrote:
> > type = "hvm"
> > memory = 1024
> > maxmem = 1073741824
> > 
> > I suspect maxmem > free Xen memory may be sufficient.  The instances I
> > can be certain of have been maxmem = total host memory *7.
> 
> Can you include your Xen version and dom0 command-line?

> This is on staging-4.14 from a month or two ago (i.e., what I happened to 
> have on a random test  box), and `dom0_mem=1024M,max:1024M` in my 
> command-line.  That rune will give dom0 only 1GiB of RAM, but also prevent it 
> from auto-ballooning down to free up memory for the guest.
> 

As this is a server, not a development target, Debian's build of 4.11 is
in use.  Your domain 0 memory allocation is extremely generous compared
to mine.  One thing which is on the command-line though is
"watchdog=true".

I've got 3 candidates which presently concern me:ble:

1> There is a limited range of maxmem values where this occurs.  Perhaps
1TB is too high on your machine for the problem to reproduce.  As
previously stated my sample configuration has maxmem being roughly 7
times actual machine memory.

2> Between issuing the `xl create` command and the machine rebooting a
few moments of slow response have been observed.  Perhaps the memory
allocator loop is hogging processor cores long enough for the watchdog to
trigger?

3> Perhaps one of the patches on Debian broke things?  This seems
unlikely since nearly all of Debian's patches are either strictly for
packaging or else picks from Xen's main branch, but this is certainly
possible.


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