Re: [RFC] mm: support multi_freearea to the reduction of external fragmentation

[David Hildenbrand <david@redhat.com>]

On 26.04.21 12:19, lipeifeng@oppo.com wrote:
> Hi David Hildenbrand <mailto:david@redhat.com>：
> 
>  >> And you don't mention what the baseline configuration was. For example,
>  >> how was compaction configured?
>  >> Just to clarify, what is monkey?
>  >> Monkey HTTP server? MonkeyTest disk benchmark? UI/Application Exerciser
>  >> Monkey?
> -------------------------------------------------------------------------------------
> I am sorry that i didn't  give a clear explanation about Monkey.
> It meant  "UI/Application Exerciser Monkey" from google.
> 
> Excuse me, let me introduce our test:
> 

Thanks for more details on the test.

> 1. record COMPACT_STALL
> We tested the patch on linux-4.4/linux-4.9/linux-4.14/linux-4.19 and the
> results shows that the patch is effective in reducing COMPACTSTALL.
>      - monkey for 12 hours.
>      - record COMPACTSTALL after test.
> 
> Test-result: reduced COMPACTSTALL by 95.6% with the patch.
> (the machine with 4 gigabytes of physical memery and in linux-4.19.)
> ---------------------------------
>                       |   COMPACTSTALL
> ---------------------------------
>     ori              |     2189
> ---------------------------------
> optimization |      95
> ---------------------------------
> 
> I fully agree with the value of compaction, but compaction also bring cpu
> consumption and will increase the time of alloc_stall. So if we can let more
> free high-orders-pages in buddy instead of signal pages, it will decrease
> COMPACT_STALL and speed up memory allocation.

Okay, but then I assume the target goal of your patch set is to minimize 
CPU consumption/allocation stall time when allocating larger order pages.

Currently you state "the probablity of high-order-pages allocation would 
be increased significantly", but I assume that's then not 100% correct. 
What you measure is the stall time to allocate higher order pages, not 
that you can allocate them.

> 
> 2. record the speed of the high-orders-pages allocation(order=4 and 
> order = 8)
> Before and after optimization, we tested the speed of the 
> high-orders-pages allocation
> after 120-hours-Monkey in 10 Android mobile phones. and the result show that
> the speed has been increased by more than 18%.
> 
> Also, we do some test designed by us:
> (the machine with 4 gigabytes of physical memery and in linux-4.19.)
> model the usage of users, and constantly start and
> operate the diffrent application for 120h, and we record COMPACT_STALL 
> is decreased by
> 90+% and speed of the high-orders-pages is increaed by 15+%.

Okay, again, this is then some optimization for allocation speed; which 
makes it less attractive IMHO (at least for more invasive changes), 
because I suspect this mostly helps in corner cases (Monkey benchmarks 
corner cases AFAIU).

> 
> and I have some question, i hope you can guide me if when you are free.
> 1) What is the compaction configured?
>      Dost it meant the members in zone? like as follows:
>      unsigned int compact_considered;
>      unsigned int compact_defer_shift;
>      int compact_order_failed;
>      bool compact_blockskip_failed;
>      Or the some Macro variable? like as follows:
>      PAGE_ALLOC_COSTLY_ORDER = 3
>      MIN_COMPACT_PRIORITY = 1
>      MAX_COMPACT_RETRIES = 16
> 

Rather if you have proactive compaction 
(/proc/sys/vm/compaction_proactiveness). But I assume because you're 
messing with older kernels, that you didn't compare against that yet. 
Would be worth a comparison.

>>> 1) multi freearea (which might
>  >> be problematic with sparcity)
> 2) Can you pls tell me what is soarcity and what is the impact of this?
>      and whether there are some documents about it?

Essentially CONFIG_SPARSEMEM, whereby we can have huge holes in physical 
memory layout and memory areas coming/going with memory hot(un)plug. 
Usually we manage all metadata per section. For example, pageblocks are 
allocated per section. We avoid arrays that depend on the 
initial/maximum physical memory size.

-- 
Thanks,

David / dhildenb