Date: Fri, 27 Mar 2009 08:17:42 +0100 From: Luigi Rizzo <rizzo@iet.unipi.it> To: Andrew Brampton <brampton+freebsd-net@gmail.com> Cc: freebsd-net@freebsd.org Subject: Re: Interrupts + Polling mode (similar to Linux's NAPI) Message-ID: <20090327071742.GA87385@onelab2.iet.unipi.it> In-Reply-To: <d41814900903261747v28d3de29t10bb1b8128de635c@mail.gmail.com> References: <d41814900903261747v28d3de29t10bb1b8128de635c@mail.gmail.com>
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On Fri, Mar 27, 2009 at 12:47:45AM +0000, Andrew Brampton wrote: > Hi, > Linux has a feature called NAPI, which amongst other things has this > Interrupt initiated polling mode. Whilst the network traffic is quiet > the network interfaces use interrupts, however as soon as the load > becomes higher polling kicks in and stays like that until the load > drops again. > > I know that FreeBSD can do interrupts or polling, but not together. I > think that that NAPI pretty neat as it provides the benefits of both > interrupts and polling, namely low CPU load (when the network is not > busy), and high performance. I was wondering if anyone has considered > this approach in FreeBSD? If not why not? Is there some reason why the > binary FreeBSD approach is better? Or is it just that no one has > dedicated the time and effort to implement this feature? The load of polling is pretty low (within 1% or so) even with polling. The advantage of having interrupts is faster response to incoming traffic, not CPU load. There is nothing difficult in having both active, except figuring out a good logic for when to disable polling on an interface that has been quiet for a while. I don't know what is the status of polling these days -- when i wrote it, the architecture was designed for UP kernels, and I don't know if/how it has been revised to deal efficiently with the SMP kernels we have now (in other words: one or multiple polling loops, interaction with interrupt threads, etc.) cheers luigi
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