On Thu, Sep 30, 2021 at 10:24:24AM -0700, Sohil Mehta wrote: > > On 9/30/2021 9:30 AM, Stefan Hajnoczi wrote: > > On Tue, Sep 28, 2021 at 09:31:34PM -0700, Andy Lutomirski wrote: > > > > > > I spent some time reviewing the docs (ISE) and contemplating how this all fits together, and I have a high level question: > > > > > > Can someone give an example of a realistic workload that would benefit from SENDUIPI and precisely how it would use SENDUIPI? Or an example of a realistic workload that would benefit from hypothetical device-initiated user interrupts and how it would use them? I'm having trouble imagining something that wouldn't work as well or better by simply polling, at least on DMA-coherent architectures like x86. > > I was wondering the same thing. One thing came to mind: > > > > An application that wants to be *interrupted* from what it's doing > > rather than waiting until the next polling point. For example, > > applications that are CPU-intensive and have green threads. I can't name > > a real application like this though :P. > > Thank you Stefan and Andy for giving this some thought. > > We are consolidating the information internally on where and how exactly we > expect to see benefits with real workloads for the various sources of User > Interrupts. It will take a few days to get back on this one. One possible use case came to mind in QEMU's TCG just-in-time compiler: QEMU's TCG threads execute translated code. There are events that require interrupting these threads. Today a check is performed at the start of every translated block. Most of the time the check is false and it's a waste of CPU. User interrupts can eliminate the need for checks by interrupting TCG threads when events occur. I don't know whether this will improve performance or how feasible it is to implement, but I've added people who might have ideas. (For a summary of user interrupts, see https://lwn.net/SubscriberLink/871113/60652640e11fc5df/.) Stefan
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