Messages in this thread | | | Subject | Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory pressure | From | Jürgen Groß <> | Date | Mon, 9 Dec 2019 11:29:19 +0100 |
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On 09.12.19 11:23, SeongJae Park wrote: > On Mon, 9 Dec 2019 10:39:02 +0100 Juergen <jgross@suse.com> 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
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