linux-kernel - Re: [PATCH 2/9] IB: add a proper completion queue abstraction

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Message-ID: <20151115125501.GB2218@lst.de>
Date:	Sun, 15 Nov 2015 13:55:01 +0100
From:	Christoph Hellwig <hch@....de>
To:	Sagi Grimberg <sagig@....mellanox.co.il>
Cc:	Christoph Hellwig <hch@....de>, linux-rdma@...r.kernel.org,
	bart.vanassche@...disk.com, axboe@...com,
	linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/9] IB: add a proper completion queue abstraction

On Sun, Nov 15, 2015 at 11:40:02AM +0200, Sagi Grimberg wrote:
> I doubt INT_MAX is useful as a budget in any use-case. it can easily
> hog the CPU. If the consumer is given access to poll a CQ, it must be
> able to provide some way to budget it. Why not expose a budget argument
> to the consumer?

Because in theory we could have a lot of sends completing before
we finally need to reap them.  I think that's more of a theoretical
than real issue.

My preference would be to simply kill this mode though.  Allocate a IU
to each block request in SRP and only use the free_tx list for task
management and AEN/req_limit calls.  Then we can use a single CQ
and mark the regular I/O requests as unsignalled.

AFAICS no other driver wants a similar polling mode as the SRP initiator
does for it's send queue.
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