From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: David Gibson <david@gibson.dropbear.id.au>
Cc: Laurent Vivier <lvivier@redhat.com>,
Michael Roth <mdroth@linux.vnet.ibm.com>,
Scott Cheloha <cheloha@linux.vnet.ibm.com>,
qemu-devel@nongnu.org, Juan Quintela <quintela@redhat.com>
Subject: Re: [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion
Date: Mon, 21 Oct 2019 09:14:44 +0100 [thread overview]
Message-ID: <20191021081444.GA2934@work-vm> (raw)
In-Reply-To: <20191019101223.GD1960@umbus.fritz.box>
* David Gibson (david@gibson.dropbear.id.au) wrote:
> On Fri, Oct 18, 2019 at 10:43:52AM +0100, Dr. David Alan Gilbert wrote:
> > * Laurent Vivier (lvivier@redhat.com) wrote:
> > > On 18/10/2019 10:16, Dr. David Alan Gilbert wrote:
> > > > * Scott Cheloha (cheloha@linux.vnet.ibm.com) 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 / dgilbert@redhat.com / Manchester, UK
next prev parent reply other threads:[~2019-10-21 8:15 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-17 20:59 [PATCH v2 0/2] migration: faster savevm_state_handler_insert() Scott Cheloha
2019-10-17 20:59 ` [PATCH v2 1/2] migration: add savevm_state_handler_remove() Scott Cheloha
2019-12-04 16:43 ` Dr. David Alan Gilbert
2020-01-08 19:07 ` Juan Quintela
2019-10-17 20:59 ` [PATCH v2 2/2] migration: savevm_state_handler_insert: constant-time element insertion Scott Cheloha
2019-10-18 8:16 ` Dr. David Alan Gilbert
2019-10-18 8:34 ` Laurent Vivier
2019-10-18 9:43 ` Dr. David Alan Gilbert
2019-10-18 16:38 ` Michael Roth
2019-10-18 17:26 ` Dr. David Alan Gilbert
2019-10-21 7:33 ` David Gibson
2019-10-19 10:12 ` David Gibson
2019-10-21 8:14 ` Dr. David Alan Gilbert [this message]
2019-11-20 21:48 ` Scott Cheloha
2019-12-04 16:49 ` Dr. David Alan Gilbert
2019-12-04 22:28 ` David Gibson
2019-12-04 16:47 ` Dr. David Alan Gilbert
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