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Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion |
Date: |
Mon, 21 Oct 2019 09:14:44 +0100 |
User-agent: |
Mutt/1.12.1 (2019-06-15) |
* David Gibson (address@hidden) wrote:
> 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.
Switch on the next machine type version - it doesn't matter if migration
is incompatible then.
Without knowing anything about the innards of DRCs, I suggest a
DRCMulti that takes a parameter and represents 'n' DRCs at consecutive
chunks of memory. Then use one DRCMulti for each RAMBlock or DIMM or
other convenient sized thing.
Dave
> --
> 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
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK
- [PATCH v2 0/2] migration: faster savevm_state_handler_insert(), Scott Cheloha, 2019/10/17
- [PATCH v2 1/2] migration: add savevm_state_handler_remove(), Scott Cheloha, 2019/10/17
- [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, Scott Cheloha, 2019/10/17
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, Dr. David Alan Gilbert, 2019/10/18
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, Laurent Vivier, 2019/10/18
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, Dr. David Alan Gilbert, 2019/10/18
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, Michael Roth, 2019/10/18
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, Dr. David Alan Gilbert, 2019/10/18
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, David Gibson, 2019/10/21
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion, David Gibson, 2019/10/19
- Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion,
Dr. David Alan Gilbert <=