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

[Huang, Ying]

Huan Yang <link@xxxxxxxx> 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 Frozen applications mean that the application process is no longer active,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 first read the file is no longer active, other processes may still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.

So, your solution may work good for your specific use cases, but it's
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).
There's some heuristics to identify use-once page caches in reclaiming
code.  Why doesn't it work for your situation?

[snip]

--
Best Regards,
Huang, Ying