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From: Sergio Lopez <slp@redhat.com>
In-reply-to: <21d96d05-b600-b115-3649-8d488aeec82b@redhat.com>
Date: Mon, 11 Mar 2019 15:35:11 +0100
Message-ID: <87y35leaxs.fsf@redhat.com>
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Subject: Re: [Qemu-devel] Flatview rendering scalability issue
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-devel@nongnu.org, Peter Xu <peterx@redhat.com>


Paolo Bonzini writes:

> On 11/03/19 14:48, Sergio Lopez wrote:
>>> The initialization is O(n^2) because the guest initializes one device at
>>> a time, so you rebuild the FlatView first with 0 devices, then 1, then
>>> 2, etc.  This is very hard to fix, if at all possible.
>>>
>>> However, each FlatView creation should be O(n) where n is the number of
>>> devices currently configured.  Please check with "info mtree -f" that
>>> you only have a fixed number of FlatViews.  Old versions had one per device.
>> I'm seeing 9 FVs with 1 PCI, and 119 with 100 PCIs.
>
> With
>
> $ eval qemu-system-x86_64 -M q35 \
>     -device\ e1000,id=n{1,2,3,4,5,6,7,8}{1,2,3}
>
> I only see 4 flat views ("system", "io", "memory", "(none)").
>
> Probably you are using intel-iommu?  Peter, it should be possible to
> reorganize the VT-d memory regions like this:

You're right, the number of FVs goes down drastically after removing
intel-iommu, and the slowness during Guest PCI initialization disappears
with it.

Thanks,
Sergio.