Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory pressure

[JÃrgen GroÃ]

On 09.12.19 11:23, SeongJae Park wrote:
> On   Mon, 9 Dec 2019 10:39:02 +0100  Juergen <jgross@xxxxxxxx> wrote:
>
> > On 09.12.19 09:58, SeongJae Park wrote:
> > > Each `blkif` has a free pages pool for the grant mapping.  The size of
> > > the pool starts from zero and be increased on demand while processing
> > > the I/O requests.  If current I/O requests handling is finished or 100
> > > milliseconds has passed since last I/O requests handling, it checks and
> > > shrinks the pool to not exceed the size limit, `max_buffer_pages`.
> > >
> > > Therefore, `blkfront` running guests can cause a memory pressure in the
> > > `blkback` running guest by attaching a large number of block devices and
> > > inducing I/O.
> >
> > I'm having problems to understand how a guest can attach a large number
> > of block devices without those having been configured by the host admin
> > before.
> >
> > If those devices have been configured, dom0 should be ready for that
> > number of devices, e.g. by having enough spare memory area for ballooned
> > pages.
>
> As mentioned in the original message as below, administrators _can_ avoid this
> problem, but finding the optimal configuration is hard, especially if the
> number of the guests is large.
>
> 	System administrators can avoid such problematic situations by limiting
> 	the maximum number of devices each guest can attach.  However, finding
> 	the optimal limit is not so easy.  Improper set of the limit can
> 	results in the memory pressure or a resource underutilization.

This sounds as if the admin would set a device limit. But it is the
other way round: The admin needs to configure each possible device
with all parameters (e.g. backing dom0 resource) for enabling the
frontend to use it.


Juergen