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Subject: Re: [PATCH 06/10] mm: vmscan: demote anon DRAM pages to PMEM node
Date:   Wed, 27 Mar 2019 13:37:46 -0700
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On 27 Mar 2019, at 11:00, Dave Hansen wrote:

> On 3/27/19 10:48 AM, Zi Yan wrote:
>> For 40MB/s vs 750MB/s, they were using sys_migrate_pages(). Sorry
>> about the confusion there. As I measure only the migrate_pages() in
>> the kernel, the throughput becomes: migrating 4KB page: 0.312GB/s
>> vs migrating 512 4KB pages: 0.854GB/s. They are still >2x
>> difference.
>>
>> Furthermore, if we only consider the migrate_page_copy() in
>> mm/migrate.c, which only calls copy_highpage() and
>> migrate_page_states(), the throughput becomes: migrating 4KB page:
>> 1.385GB/s vs migrating 512 4KB pages: 1.983GB/s. The gap is
>> smaller, but migrating 512 4KB pages still achieves 40% more
>> throughput.
>>
>> Do these numbers make sense to you?
>
> Yes.  It would be very interesting to batch the migrations in the
> kernel and see how it affects the code.  A 50% boost is interesting,
> but not if it's only in microbenchmarks and takes 2k lines of code.
>
> 50% is *very* interesting if it happens in the real world and we can
> do it in 10 lines of code.
>
> So, let's see what the code looks like.

Actually, the migration throughput difference does not come from any kern=
el
changes, it is a pure comparison between migrate_pages(single 4KB page) a=
nd
migrate_pages(a list of 4KB pages). The point I wanted to make is that
Yang=E2=80=99s approach, which migrates a list of pages at the end of shr=
ink_page_list(),
can achieve higher throughput than Keith=E2=80=99s approach, which migrat=
es one page
at a time in the while loop inside shrink_page_list().

In addition to the above, migrating a single THP can get us even higher t=
hroughput.
Here is the throughput numbers comparing all three cases:
                             |  migrate_page()  |    migrate_page_copy()
migrating single 4KB page:   |  0.312GB/s       |   1.385GB/s
migrating 512 4KB pages:     |  0.854GB/s       |   1.983GB/s
migrating single 2MB THP:    |  2.387GB/s       |   2.481GB/s

Obviously, we would like to migrate THPs as a whole instead of 512 4KB pa=
ges
individually. Of course, this assumes we have free space in PMEM for THPs=
 and
all subpages in the THPs are cold.


To batch the migration, I posted some code a while ago: https://lwn.net/A=
rticles/714991/,
which show good throughput improvement for microbenchmarking sys_migrate_=
page().
It also included using multi threads to copy a page, aggregate multiple m=
igrate_page_copy(),
and even using DMA instead of CPUs to copy data. We could revisit the cod=
e if necessary.

In terms of end-to-end results, I also have some results from my paper:
http://www.cs.yale.edu/homes/abhishek/ziyan-asplos19.pdf (Figure 8 to Fig=
ure 11 show the
microbenchmark result and Figure 12 shows end-to-end results). I basicall=
y called
shrink_active/inactive_list() every 5 seconds to track page hotness and u=
sed all my page
migration optimizations above, which can get 40% application runtime spee=
dup on average.
The experiments were done in a two-socket NUMA machine where one node was=
 slowed down to
have 1/2 BW and 2x access latency, compared to the other node. I can disc=
uss about it
more if you are interested.


--
Best Regards,
Yan Zi

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