Re: [RFC PATCH 00/25] Accelerate page migration and use memcg for PMEM management

[Zi Yan]

>> Infrequent page list update problem
>> ====
>>
>> Current page lists are updated by calling shrink_list() when memory pressure
>> comes,  which might not be frequent enough to keep track of hot and cold pages.
>> Because all pages are on active lists at the first time shrink_list() is called
>> and the reference bit on the pages might not reflect the up to date access status
>> of these pages. But we also do not want to periodically shrink the global page
>> lists, which adds unnecessary overheads to the whole system. So I propose to
>> actively shrink page lists on the memcg we are interested in.
>>
>> Patch 18 to 25 add a new system call to shrink page lists on given application's
>> memcg and migrate pages between two NUMA nodes. It isolates the impact from the
>> rest of the system. To share DRAM among different applications, Patch 18 and 19
>> add per-node memcg size limit, so you can limit the memory usage for particular
>> NUMA node(s).
>
> This sounds a little bit confusing to me. Is it totally user's decision about when to call the syscall to shrink page lists? But, how would user know when is a good timing? Could you please elaborate the usecase?

Sure. We would set up a daemon that monitors user applications and calls the syscall
to shuffle the page lists for the user applications, although the daemonâs concrete
action plan is still under exploration. It might not be ideal but the page access information
could be refreshed periodically and page migration would happen on the background of
application execution.

On the other hand, if we wait until DRAM is full and use page migration to make room in DRAM
for either page promotion or new page allocation, page migration sits on the critical path
of application execution. Considering the bandwidth and access latency gaps between
DRAM and PMEM are not as large as the gaps between DRAM and SSD, the cost of page migration
(4KB/0.312GB/s = 12us or 2MB/2.387GB/s = 818us)might defeat the benefit of using DRAM over PMEM.
I just wonder which would be better: waiting for 12us or 818us then reading 4KB or 2MB data in DRAM
or directly accessing the data in PMEM without waiting.

Let me know if this makes sense to you.

Thanks.

--
Best Regards,
Yan Zi

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