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Date:   Tue, 11 Sep 2018 11:21:26 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Sumit Saxena <sumit.saxena@broadcom.com>
Cc:     Ming Lei <ming.lei@redhat.com>, tglx@linutronix.de, hch@lst.de,
        linux-kernel@vger.kernel.org,
        Kashyap Desai <kashyap.desai@broadcom.com>,
        Shivasharan Srikanteshwara 
        <shivasharan.srikanteshwara@broadcom.com>
Subject: Re: Affinity managed interrupts vs non-managed interrupts
Message-ID: <20180911092126.GA10330@lst.de>
References: <eccc46e12890a1d033d9003837012502@mail.gmail.com> <20180829084618.GA24765@ming.t460p> <300d6fef733ca76ced581f8c6304bac6@mail.gmail.com>
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On Wed, Aug 29, 2018 at 04:16:23PM +0530, Sumit Saxena wrote:
> > Could you explain a bit what the specific use case the extra 16 vectors
> is?
> We are trying to avoid the penalty due to one interrupt per IO completion
> and decided to coalesce interrupts on these extra 16 reply queues.
> For regular 72 reply queues, we will not coalesce interrupts as for low IO
> workload, interrupt coalescing may take more time due to less IO
> completions.
> In IO submission path, driver will decide which set of reply queues
> (either extra 16 reply queues or regular 72 reply queues) to be picked
> based on IO workload.

The point I don't get here is why you need separate reply queues for
the interrupt coalesce setting.  Shouldn't this just be a flag at
submission time that indicates the amount of coalescing that should
happen?

What is the benefit of having different completion queues?