On Thu, Sep 24, 2020 at 12:27:14PM +0200, SeongJae Park wrote: > On Thu, 24 Sep 2020 12:13:44 +0200 "Roger Pau Monné" <roger.pau@xxxxxxxxxx> wrote: > > > 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@xxxxxxxxx> > > > > > > > > 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. > > Agreed. > > > > > 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. > > I think we could extend this design by receiving list of exceptional domains. > For example, if 'feature_persistent' is True and exceptions list has '123, > 456', domains of domid 123 and 456 will not use persistent grants, and vice > versa. I think that would be quite fragile IMO, I wouldn't recommend relying on domain IDs. What I would do instead is add a new attribute to xl-disk-configuration [0] that allows setting the persistent grants usage on a per-disk basis, and that should be passed to blkback in a xenstore node. > I could implement this, but... to be honest, I don't really understand the > needs of the fine-grained control. AFAIU, the problem is 'scalability' vs > 'data copy overhead'. So, only small systems would want to turn persistent > grants off. In such a small system, why would we need fine-grained control? > I'm worrying if I would implement and maintain a feature without real use case. > > For the reason, I'd like to suggest to keep this as is for now and expand it > with the 'exceptions list' idea or something better, if a real use case comes > out later. I agree. I'm happy to take patches to implement more fine grained control, but that shouldn't prevent us from having a global policy if that's useful to users. Roger. [0] https://xenbits.xen.org/docs/unstable/man/xl-disk-configuration.5.html
