linux-kernel - Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier interface

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Date:	Thu, 18 Jun 2009 10:44:02 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	avi@...hat.com, paulmck@...ux.vnet.ibm.com,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [KVM-RFC PATCH 1/2] eventfd: add an explicit srcu based notifier
 interface

On Thu, 18 Jun 2009, Gregory Haskins wrote:

> Actually there is only one (the tx-thread) aside from the eventfd
> imposed workqueue one.  Incidentally, I would love to get rid of the
> other thread too, so I am not just picking on eventfd ;).  The other is
> a lot harder since it has to update the virtio-ring and may need to page
> in guest memory to do so.

No, there is the interface rx softirq too, that makes two. Plus, the 
process of delivering (especially for KVM & Co.) does not involve only ctx 
switching, there's other stuff in the middle too.
You also talk about latency. Really? Probably RT people aren't looking 
into KVM if RT is even a mild requirement.



> To flip it around on you: try telling a group like the netdev guys that
> they should put extra context switches into the stack because they don't
> really matter.  Be sure to wear extra thick asbestos. ;)

They already do. So you've got to ask yourself why they can achieve Gbps 
throughput already, why can't KVM live with it and being compelled to 
litter an existing interface.



> The fact is that eventfd is a really neat general signaling idiom. 
> However, its currently geared towards "signaling = wakeup".  As we have
> proven with this KVM *fd effort, this is not necessarily accurate to
> describe all use cases, nor is it optimal.  I'd like to address that. 
> An alternative, of course, is that we make a private anon-fd solution
> within KVM.  However, it will be so similar to eventfd so it just seems
> wasteful if it can be avoided.

Even though you take that route, you'll have to prove with replicable real 
life benchmarks, that the bloating make sense.



- Davide


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