[Qemu-devel] Re: [PATCH 09/10] Exit loop if we have been there too long

[Avi Kivity]

On 11/30/2010 03:47 PM, Anthony Liguori wrote:
> On 11/30/2010 01:15 AM, Paolo Bonzini wrote:
> > On 11/30/2010 03:11 AM, Anthony Liguori wrote:
> > > BufferedFile should hit the qemu_file_rate_limit check when the socket
> > > buffer gets filled up.
> >
> > The problem is that the file rate limit is not hit because work is 
> > done elsewhere.  The rate can limit the bandwidth used and makes QEMU 
> > aware that socket operations may block (because that's what the 
> > buffered file freeze/unfreeze logic does); but it cannot be used to 
> > limit the _time_ spent in the migration code.
>
> Yes, it can, if you set the rate limit sufficiently low.
>
> The caveats are 1) the kvm.ko interface for dirty bits doesn't scale 
> for large memory guests so we spend a lot more CPU time walking it 
> than we should 2) zero pages cause us to burn a lot more CPU time than 
> we otherwise would because compressing them is so effective.

What's the problem with burning that cpu?  per guest page, compressing 
takes less than sending.  Is it just an issue of qemu mutex hold time?

> In the short term, fixing (2) by accounting zero pages as full sized 
> pages should "fix" the problem.
>
> In the long term, we need a new dirty bit interface from kvm.ko that 
> uses a multi-level table.  That should dramatically improve scan 
> performance. 

Why would a multi-level table help?  (or rather, please explain what you 
mean by a multi-level table).

Something we could do is divide memory into more slots, and polling each 
slot when we start to scan its page range.  That reduces the time 
between sampling a page's dirtiness and sending it off, and reduces the 
latency incurred by the sampling.  There are also non-interface-changing 
ways to reduce this latency, like O(1) write protection, or using dirty 
bits instead of write protection when available.

> We also need to implement live migration in a separate thread that 
> doesn't carry qemu_mutex while it runs.

IMO that's the biggest hit currently.

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