linux-kernel - Re: [PATCH] xen-blkback: add a parameter for disabling of persistent grants

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Message-ID: <20200924101344.GN19254@Air-de-Roger>
Date:   Thu, 24 Sep 2020 12:13:44 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:     SeongJae Park <sjpark@...zon.com>,
        SeongJae Park <sjpark@...zon.de>, <axboe@...nel.dk>,
        <aliguori@...zon.com>, <amit@...nel.org>, <mheyne@...zon.de>,
        <linux-block@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen-blkback: add a parameter for disabling of persistent
 grants

On Wed, Sep 23, 2020 at 04:09:30PM -0400, Konrad Rzeszutek Wilk wrote:
> On Tue, Sep 22, 2020 at 09:01:25AM +0200, SeongJae Park wrote:
> > From: SeongJae Park <sjpark@...zon.de>
> > 
> > Persistent grants feature provides high scalability.  On some small
> > systems, however, it could incur data copy overhead[1] and thus it is
> > required to be disabled.  But, there is no option to disable it.  For
> > the reason, this commit adds a module parameter for disabling of the
> > feature.
> 
> Would it be better suited to have it per guest?

I think having a per-backend policy that could be specified at the
toolstack level would be nice, but I see that as a further
improvement.

Having a global backend domain policy of whether persistent grants are
enabled or not seems desirable, and if someone wants even more fine
grained control this change is AFAICT not incompatible with a
per-backend option anyway.

Roger.