linux-kernel - Re: [RFC 0/4] Introduce unbalance proactive reclaim

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Message-ID: <87a5rif58s.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date:   Mon, 13 Nov 2023 16:05:07 +0800
From:   "Huang, Ying" <ying.huang@...el.com>
To:     Huan Yang <link@...o.com>
Cc:     Michal Hocko <mhocko@...e.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        "Jonathan Corbet" <corbet@....net>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        "Shakeel Butt" <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        "Andrew Morton" <akpm@...ux-foundation.org>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Kefeng Wang <wangkefeng.wang@...wei.com>,
        Peter Xu <peterx@...hat.com>,
        "Vishal Moola (Oracle)" <vishal.moola@...il.com>,
        Yosry Ahmed <yosryahmed@...gle.com>,
        "Liu Shixin" <liushixin2@...wei.com>,
        Hugh Dickins <hughd@...gle.com>, <cgroups@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linux-mm@...ck.org>, <opensource.kernel@...o.com>
Subject: Re: [RFC 0/4] Introduce unbalance proactive reclaim

Huan Yang <link@...o.com> writes:

> 在 2023/11/13 14:10, Huang, Ying 写道:
>> Huan Yang <link@...o.com> writes:
>>
>>> 在 2023/11/10 20:24, Michal Hocko 写道:
>>>> On Fri 10-11-23 11:48:49, Huan Yang wrote:
>>>> [...]
>>>>> Also, When the application enters the foreground, the startup speed
>>>>> may be slower. Also trace show that here are a lot of block I/O.
>>>>> (usually 1000+ IO count and 200+ms IO Time) We usually observe very
>>>>> little block I/O caused by zram refault.(read: 1698.39MB/s, write:
>>>>> 995.109MB/s), usually, it is faster than random disk reads.(read:
>>>>> 48.1907MB/s write: 49.1654MB/s). This test by zram-perf and I change a
>>>>> little to test UFS.
>>>>>
>>>>> Therefore, if the proactive reclamation encounters many file pages,
>>>>> the application may become slow when it is opened.
>>>> OK, this is an interesting information. From the above it seems that
>>>> storage based IO refaults are order of magnitude more expensive than
>>>> swap (zram in this case). That means that the memory reclaim should
>>>> _in general_ prefer anonymous memory reclaim over refaulted page cache,
>>>> right? Or is there any reason why "frozen" applications are any
>>>> different in this case?
>>> Frozen applications mean that the application process is no longer active,
>>> so once its private anonymous page data is swapped out, the anonymous
>>> pages will not be refaulted until the application becomes active again.
>>>
>>> On the contrary, page caches are usually shared. Even if the
>>> application that
>>> first read the file is no longer active, other processes may still
>>> read the file.
>>> Therefore, it is not reasonable to use the proactive reclamation
>>> interface to
>>> reclaim page caches without considering memory pressure.
>> No.  Not all page caches are shared.  For example, the page caches used
>> for use-once streaming IO.  And, they should be reclaimed firstly.
> Yes, but this part is done very well in MGLRU and does not require our
> intervention.
> Moreover, the reclaim speed of clean files is very fast, but compared to it,
> the reclaim speed of anonymous pages is a bit slower.
>>
>> So, your solution may work good for your specific use cases, but it's
> Yes, this approach is not universal.
>> not a general solution.  Per my understanding, you want to reclaim only
>> private pages to avoid impact the performance of other applications.
>> Privately mapped anonymous pages is easy to be identified (And I suggest
>> that you can find a way to avoid reclaim shared mapped anonymous pages).
> Yes, it is not good to reclaim shared anonymous pages, and it needs to be
> identified. In the future, we will consider how to filter them.
> Thanks.
>> There's some heuristics to identify use-once page caches in reclaiming
>> code.  Why doesn't it work for your situation?
> As mentioned above, the default reclaim algorithm is suitable for recycling
> file pages, but we do not need to intervene in it.
> Direct reclaim or kswapd of these use-once file pages is very fast and will
> not cause lag or other effects.
> Our overall goal is to actively and reasonably compress unused anonymous
> pages based on certain strategies, in order to increase available memory to
> a certain extent, avoid lag, and prevent applications from being killed.
> Therefore, using the proactive reclaim interface, combined with LRU
> algorithm
> and reclaim tendencies, is a good way to achieve our goal.

If so, why can't you just use the proactive reclaim with some large
enough swappiness?  That will reclaim use-once page caches and compress
anonymous pages.  So, more applications can be kept in memory before
passive reclaiming or killing background applications?

--
Best Regards,
Huang, Ying