Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion

[David Gibson]

On Fri, Oct 18, 2019 at 10:43:52AM +0100, Dr. David Alan Gilbert wrote:
> * Laurent Vivier (address@hidden) wrote:
> > On 18/10/2019 10:16, Dr. David Alan Gilbert wrote:
> > > * Scott Cheloha (address@hidden) wrote:
> > >> savevm_state's SaveStateEntry TAILQ is a priority queue.  Priority
> > >> sorting is maintained by searching from head to tail for a suitable
> > >> insertion spot.  Insertion is thus an O(n) operation.
> > >>
> > >> If we instead keep track of the head of each priority's subqueue
> > >> within that larger queue we can reduce this operation to O(1) time.
> > >>
> > >> savevm_state_handler_remove() becomes slightly more complex to
> > >> accomodate these gains: we need to replace the head of a priority's
> > >> subqueue when removing it.
> > >>
> > >> With O(1) insertion, booting VMs with many SaveStateEntry objects is
> > >> more plausible.  For example, a ppc64 VM with maxmem=8T has 40000 such
> > >> objects to insert.
> > > 
> > > Separate from reviewing this patch, I'd like to understand why you've
> > > got 40000 objects.  This feels very very wrong and is likely to cause
> > > problems to random other bits of qemu as well.
> > 
> > I think the 40000 objects are the "dr-connectors" that are used to plug
> > peripherals (memory, pci card, cpus, ...).
> 
> Yes, Scott confirmed that in the reply to the previous version.
> IMHO nothing in qemu is designed to deal with that many devices/objects
> - I'm sure that something other than the migration code is going to
> get upset.

It kind of did.  Particularly when there was n^2 and n^3 cubed
behaviour in the property stuff we had some ludicrously long startup
times (hours) with large maxmem values.

Fwiw, the DRCs for PCI slots, DRCs and PHBs aren't really a problem.
The problem is the memory DRCs, there's one for each LMB - each 256MiB
chunk of memory (or possible memory).

> Is perhaps the structure wrong somewhere - should there be a single DRC
> device that knows about all DRCs?

Maybe.  The tricky bit is how to get there from here without breaking
migration or something else along the way.

-- 
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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