linux-kernel - Re: Affinity managed interrupts vs non-managed interrupts

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Message-ID: <20180911092126.GA10330@lst.de>
Date:   Tue, 11 Sep 2018 11:21:26 +0200
From:   Christoph Hellwig <hch@....de>
To:     Sumit Saxena <sumit.saxena@...adcom.com>
Cc:     Ming Lei <ming.lei@...hat.com>, tglx@...utronix.de, hch@....de,
        linux-kernel@...r.kernel.org,
        Kashyap Desai <kashyap.desai@...adcom.com>,
        Shivasharan Srikanteshwara 
        <shivasharan.srikanteshwara@...adcom.com>
Subject: Re: Affinity managed interrupts vs non-managed interrupts

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?