Re: [Qemu-devel] [RFC PATCH qemu 0/4] memory: Reduce memory use

[David Gibson <david@gibson.dropbear.id.au>]

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On Thu, Sep 07, 2017 at 10:51:42AM +0100, Dr. David Alan Gilbert wrote:
> * Alexey Kardashevskiy (aik@ozlabs.ru) wrote:
> > This was inspired by https://bugzilla.redhat.com/show_bug.cgi?id=1481593
> > 
> > What happens ithere is that every virtio block device creates 2 address
> > spaces - for modern config space (called "virtio-pci-cfg-as") and
> > for busmaster (common pci thing, called after the device name,
> > in my case "virtio-blk-pci").
> > 
> > Each address_space_init() updates topology for every address space.
> > Every topology update (address_space_update_topology()) creates a new
> > dispatch tree - AddressSpaceDispatch with nodes (1KB) and
> > sections (48KB) and destroys the old one.
> > 
> > However the dispatch destructor is postponed via RCU which does not
> > get a chance to execute until the machine is initialized but before
> > we get there, memory is not returned to the pool, and this is a lot
> > of memory which grows n^2.
> > 
> > These patches are trying to address the memory use and boot time
> > issues but tbh only the first one provides visible outcome.
> 
> Do you have a feel for how much memory is saved?

I think that's a bit hard to answer.

As noted above there's O(n^2) (or more) space complexity here - one
which shouldn't be required by the data we actually have to track.
That means the amount of "excess" memory depends on how many devices
there are.

I haven't yet looked at these patches in detail, to know if it truly
fixes that O(n^2) or just pares the constant.  If it does fix the
O(n^2) then the amount is going to vary from "probably not enough to
worry about" in normal use cases to hundreds of gigabytes in cases
with many devices.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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