[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qem

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qemu
Date: Fri, 06 Nov 2009 10:09:23 -0600
User-agent: Thunderbird (X11/20090825)

Avi Kivity wrote:
On 11/06/2009 02:29 AM, Anthony Liguori wrote:
Helpers are really bad. On launch, I find the fragile and hard to do proper error handling with (but that's probably just me). But the real problem is at runtime, if you have a 16GB guest then you have to write-protect 4M ptes and then kvm has to tear down or write protect (not sure which mmu notifier is called) 4M shadow ptes. Once that's done, the guest will have to fault its way back; that's at least 4M exits, around 10 seconds worth of cpu time to execute a couple of syscalls.

FWIW, with large pages, it's only 8k exits which comes out to about 16ms. If nothing else, it's an argument for the important of transparent large pages.

No, it's an argument against fork() of large programs.

After putting together a work around, I'm starting to have my doubts about how real of a problem this is.

You're only write protecting memory, correct? So it's equivalent to enabling dirty tracking during live migration. In my mind, if the cost associated with hot plug is a fraction of the cost of live migration, we're in good shape.

It's not likely that a 16GB guest is going to write-fault in it's entirely memory range immediately. In fact, it's likely to be amortized over a very long period of time so I have a hard time believing this is really an issue in practice.

Arguably, it's a much bigger problem for live migration.


Anthony Liguori

reply via email to

[Prev in Thread] Current Thread [Next in Thread]