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Subject: Re: [PATCH v3 0/1] xen/blkback: Squeeze page pools if a memory
 pressure
To:     SeongJae Park <sjpark@amazon.com>
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        pdurrant@amazon.com, sj38.park@gmail.com,
        xen-devel@lists.xenproject.org
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 <20191209102347.17337-1-sjpark@amazon.com>
From:   =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>
Message-ID: <7a1dfa22-1108-602f-68ff-ed30a18c1d3d@suse.com>
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