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Re: [Xen-devel] Xen 4.0.1 "xc_map_foreign_batch: mmap failed: Cannot all

To: "Stefano Stabellini" <stefano.stabellini@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Xen 4.0.1 "xc_map_foreign_batch: mmap failed: Cannot allocate memory"
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Wed, 05 Jan 2011 15:30:53 +0000
Cc: Charles Arnold <CARNOLD@xxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Keir Fraser <keir@xxxxxxx>
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>>> On 05.01.11 at 15:37, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> On Thu, 16 Dec 2010, Keir Fraser wrote:
>> On 16/12/2010 20:44, "Charles Arnold" <carnold@xxxxxxxxxx> wrote:
>> >>> On 12/16/2010 at 01:33 PM, in message <C9302813.2966F%keir@xxxxxxx>, Keir
>> > Fraser <keir@xxxxxxx> wrote:
>> >> On 16/12/2010 19:23, "Charles Arnold" <carnold@xxxxxxxxxx> wrote:
>> >> 
>> >>> The bug is that qemu-dm seems to make the assumption that it can mmap 
>> >>> from
>> >>> dom0 all the memory with which the guest has been defined instead of the
>> >>> memory
>> >>> that is actually available on the host.
>> >> 
>> >> 32-bit dom0? Hm, I thought the qemu mapcache was supposed to limit the 
>> >> total
>> >> amount of guest memory mapped at one time, for a 32-bit qemu. For 64-bit
>> >> qemu I wouldn't expect to find a limit as low as 3.25G.
>> > 
>> > Sorry, I should have specified that it is a 64 bit dom0 / hypervisor.
>> Okay, well I'm not sure what limit qemu-dm is hitting then. Mapping 3.25G of
>> guest memory will only require a few megabytes of pagetables for the qemu
>> process in dom0. Perhaps there is a ulimit or something set on the qemu
>> process?
>> If we can work out and detect this limit, perhaps 64-bit qemu-dm could have
>> a mapping cache similar to 32-bit qemu-dm, limited to some fraction of the
>> detected mapping limit. And/or, on mapping failure, we could reclaim
>> resources by simply zapping the existing cached mappings. Seems there's a
>> few options. I don't really maintain qemu-dm myself -- you might get some
>> help from Ian Jackson, Stefano, or Anthony Perard if you need more advice.
> The mapcache size limit should be 64GB on a 64bit qemu-dm.
> Any interesting error messages in the qemu logs?

Despite knowing next to nothing about qemu, I'm not certain the
mapcache alone matters here: One would expect this to only
consume memory for page table construction, but then you
wouldn't need Dom0 to have more memory than the guest for the
latter to do heavy I/O. There ought to be something that
allocates memory in amounts roughly equivalent to what the
guest has under I/O.


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