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