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From: Laurent Vivier <lvivier@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>,
	Scott Cheloha <cheloha@linux.vnet.ibm.com>,
	david@gibson.dropbear.id.au
Cc: Michael Roth <mdroth@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: Fri, 18 Oct 2019 10:34:47 +0200
Message-ID: <351dca8e-e77c-c450-845b-d78ba621156a@redhat.com> (raw)
In-Reply-To: <20191018081625.GA2990@work-vm>

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, ...).

https://github.com/qemu/qemu/blob/master/hw/ppc/spapr_drc.c

They are part of SPAPR specification.

https://raw.githubusercontent.com/qemu/qemu/master/docs/specs/ppc-spapr-hotplug.txt

CC Michael Roth

Thanks,
Laurent


  reply index

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 [this message]
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
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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