linux-kernel - Re: [ckrm-tech] RFC: Memory Controller

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Message-ID: <45472133.9090109@in.ibm.com>
Date:	Tue, 31 Oct 2006 15:40:59 +0530
From:	Balbir Singh <balbir@...ibm.com>
To:	Pavel Emelianov <xemul@...nvz.org>
CC:	vatsa@...ibm.com, dev@...nvz.org, sekharan@...ibm.com,
	ckrm-tech@...ts.sourceforge.net, haveblue@...ibm.com,
	linux-kernel@...r.kernel.org, pj@....com, matthltc@...ibm.com,
	dipankar@...ibm.com, rohitseth@...gle.com, menage@...gle.com,
	linux-mm@...ck.org, Vaidyanathan S <svaidy@...ibm.com>
Subject: Re: [ckrm-tech] RFC: Memory Controller

Pavel Emelianov wrote:
> Balbir Singh wrote:
>> Pavel Emelianov wrote:
>>> [snip]
>>>
>>>>> But in general I agree, these are the three important resources for
>>>>> accounting and control
>>>> I missed out to mention, I hope you were including the page cache in
>>>> your definition of reclaimable memory.
>>> As far as page cache is concerned my opinion is the following.
>>> (If I misunderstood you, please correct me.)
>>>
>>> Page cache is designed to keep in memory as much pages as
>>> possible to optimize performance. If we start limiting the page
>>> cache usage we cut the performance. What is to be controlled is
>>> _used_ resources (touched pages, opened file descriptors, mapped
>>> areas, etc), but not the cached ones. I see nothing bad if the
>>> page that belongs to a file, but is not used by ANY task in BC,
>>> stays in memory. I think this is normal. If kernel wants it may
>>> push this page out easily it won't event need to try_to_unmap()
>>> it. So cached pages must not be accounted.
>>>
>> The idea behind limiting the page cache is this
>>
>> 1. Lets say one container fills up the page cache.
>> 2. The other containers will not be able to allocate memory (even
>> though they are within their limits) without the overhead of having
>> to flush the page cache and freeing up occupied cache. The kernel
>> will have to pageout() the dirty pages in the page cache.
>>
>> Since it is easy to push the page out (as you said), it should be
>> easy to impose a limit on the page cache usage of a container.
> 
> If a group is limited with memory _consumption_ it won't fill
> the page cache...
> 

So you mean the memory _consumption_ limit is already controlling
the page cache? That's what we need the ability for a container
not to fill up the page cache :)

I don't remember correctly, but do you account for dirty page cache usage in
the latest patches of BC?

>>> I've also noticed that you've [snip]-ed on one of my questions.
>>>
>>>  > How would you allocate memory on NUMA in advance?
>>>
>>> Please, clarify this.
>> I am not quite sure I understand the question. Could you please rephrase
>> it and highlight some of the difficulty?
> 
> I'd like to provide a guarantee for a newly created group. According
> to your idea I have to preallocate some pages in advance. OK. How to
> select a NUMA node to allocate them from?

The idea of pre-allocation was discussed as a possibility in the case
that somebody needed hard guarantees, but most of us don't need it.
I was in the RFC for the sake of completeness.

Coming back to your question

Why do you need to select a NUMA node? For performance?

-- 

	Balbir Singh,
	Linux Technology Center,
	IBM Software Labs
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