Re: [Qemu-devel] Question about vNVDIMM file format

[Richard W.M. Jones]

On Wed, May 18, 2016 at 03:04:52PM +0800, Xiao Guangrong wrote:
> On 05/17/2016 02:25 AM, Richard W.M. Jones wrote:
> >(a) How necessary is the ACPI dependency?  We disable ACPI because it
> >is quite slow, adding something like 150-200ms to the boot process
> >(every millisecond counts for us!).  Because I previously never needed
> >ACPI, I never really looked into why this is, and it could be
> >something quite simple, so I'm going to look at this issue next.  I
> >understand that NVDIMMs are not regular (eg) PCI devices, so ordinary
> >device probing isn't going to work, and that probably answers the
> >question why you need to use ACPI.
> 
> Yes, ACPI is necessary to export NVDIMM devices. The good news is that
> Intel is working on ‘lite QEMU’ which only has basic/simplest ACPI
> support. Haozhong, who has been CCed, is working on it.

I remeasured the ACPI overhead with the latest upstream kernel & qemu,
it has dropped to under 20ms, so now I've just unconditionally enabled
ACPI.

> >(c) I've got the root filesystem (which is actually ext2, but using
> >the ext4.ko driver) mounted with -o dax.  What benefits / differences
> >should I observe?  Just general reduced memory / page cache usage?
> >
> 
> And better performance as slow IO path is not needed anymore. :)
> 
> However, there is potential issue if it is not backend by real NVDIMM
> hardware, the data is not persistent. We are going to resolve it by
> emulating PCOMMIT and do msync properly.

I'm using share=off (ie. MMAP_PRIVATE), because for this appliance
model I don't want writes to go to the backing disk.

Rich.

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