Re: [PULL 15/18] qapi: introduce x-query-ramblock QMP command

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: "Daniel P. Berrangé" <berrange@redhat.com>
Cc: "Claudio Fontana" <cfontana@suse.de>,
	qemu-devel@nongnu.org, "Laurent Vivier" <lvivier@redhat.com>,
	"Thomas Huth" <thuth@redhat.com>,
	"Eduardo Habkost" <ehabkost@redhat.com>,
	"David Hildenbrand" <david@redhat.com>,
	"Michael Roth" <michael.roth@amd.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Yuval Shaia" <yuval.shaia.ml@gmail.com>,
	"Peter Xu" <peterx@redhat.com>,
	"Gerd Hoffmann" <kraxel@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Philippe Mathieu-Daudé" <philmd@redhat.com>,
	"Eric Blake" <eblake@redhat.com>,
	"Markus Armbruster" <armbru@redhat.com>
Subject: Re: [PULL 15/18] qapi: introduce x-query-ramblock QMP command
Date: Thu, 9 Jun 2022 13:52:28 +0100	[thread overview]
Message-ID: <YqHtDN/+DNUpB/ve@work-vm> (raw)
In-Reply-To: <YqHJKgA5OG80dljx@redhat.com>

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Thu, Jun 09, 2022 at 12:07:31PM +0200, Claudio Fontana wrote:
> > Hello all,
> > 
> > it would be really good to be able to rely on this command or something similar,
> > to be able to know the approximate size of a migration before starting it.
> > 
> > in QEMU ram_bytes_total() returns what I would like to have,
> > but there is currently no QMP way to get it without starting a migration,
> > which when trying to optimize it/size it is just about too late.
> 
> Aside from the main VM RAM, what other RAM blocks are likely to have
> a size large enough to be of consequence to the live migration
> data copy, and whose size is not already known to the mgmt app from
> the guest config choices it made ? VGA RAM could be a few 100MB I
> guess, but the mgmt app knows about that. I've always assumed everything
> else is just noise in comparison to the main RAM region.
> 
> Still I wonder how useful this is as its just a static figure, and the
> problems with migration transfer are the bulking up of data when the
> VM is repeatedly dirtying stuff at a high rate.
> 
> > Do you think x-query-ramblock could be promoted to non-experimental?
> 
> It would have to be re-written, as this current impl is just emitting
> a huge printf formatted string. To be considered supportable, the data
> would have to be formally modelled in QAPI instead.
> 
> IOW, it would be a case of introducing a new command that emits formal
> data, convertintg 'info ramblock' to use that, and then deprecating this 
> x-query-ramblock.
> 
> > Should another one be made available instead, like :
> > query-ram-bytes-total ?
> 
> That would be simpler if you're just wanting it to give a single
> figure.

Is this what qmp_query_memory_size_summary does?

Dave

> 
> With regards,
> Daniel
> -- 
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK