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From: Anthony Liguori <anthony@codemonkey.ws>
To: dlaor@redhat.com
Cc: Glauber Costa <glommer@redhat.com>, Uri Lublin <uril@redhat.com>,
	qemu-devel@nongnu.org
Subject: Re: [Qemu-devel] [PATCH] ram_save_live: add a no-progress convergence rule
Date: Tue, 19 May 2009 13:15:32 -0500	[thread overview]
Message-ID: <4A12F744.3080809@codemonkey.ws> (raw)
In-Reply-To: <4A12C942.4090708@redhat.com>

Dor Laor wrote: 
> The problem is that if migration is not progressing since the guest is 
> dirtying pages
> faster than the migration protocol can send, than we just waist time 
> and cpu.
> The minimum is to notify the monitor interface in order to let mgmt 
> daemon to trap it.
> We can easily see this issue while running iperf in the guest or any 
> other high load/dirty
> pages scenario.

The problem is, what's the metric for determining the guest isn't 
progressing?  A raw iteration count is not a valid metric.  It may be 
expected that the migration take 50 iterations.

The management tool knows the guest isn't progressing when it decides 
that a guest isn't progressing :-)

> We can also make it configurable using the monitor migrate command. 
> For example:
> migrate -d -no_progress -threshold=x tcp:....

Theshold is really a bad metric to use.  You have no idea how much data 
has been passed in each iteration.  If you only needed one more 
iteration, then stopping the migration short was a really bad idea.

The only thing that this does is give a false sense of security.  
Management tools have to deal with forcing migration convergence based 
on policies.  If a management tool isn't doing this today, it's broken IMHO.

Basically, threshold introduces a regression.  If you run iperf and 
migrate a guest with a very large memory size, after migration, you'll 
get soft lockups because the guest hasn't been running for 10 seconds.  
This is bad.

Regards,

Anthony Liguori

  parent reply	other threads:[~2009-05-19 18:15 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2009-05-19 11:09 [Qemu-devel] [PATCH] ram_save_live: add a no-progress convergence rule Uri Lublin
2009-05-19 13:00 ` Anthony Liguori
2009-05-19 14:41   ` Glauber Costa
2009-05-19 14:59     ` Dor Laor
2009-05-19 15:09       ` Glauber Costa
2009-05-19 18:17         ` Anthony Liguori
2009-05-20 16:56           ` Uri Lublin
2009-05-20 17:28             ` Blue Swirl
2009-05-20 17:34               ` Uri Lublin
2009-05-19 18:19         ` Anthony Liguori
2009-05-19 18:15       ` Anthony Liguori [this message]
2009-05-20 17:17         ` Uri Lublin
2009-05-19 18:09     ` Anthony Liguori
2009-05-20 17:25       ` Uri Lublin
2009-05-20 17:15     ` Daniel P. Berrange

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