linux-kernel - Re: [RFC PATCH v4 00/13] ktask: multithread CPU-intensive kernel work

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Message-ID: <7E53DD63-4955-480D-8C0D-EB07E4FF011B@cs.rutgers.edu>
Date:   Mon, 05 Nov 2018 21:48:56 -0500
From:   "Zi Yan" <zi.yan@...rutgers.edu>
To:     "Daniel Jordan" <daniel.m.jordan@...cle.com>
Cc:     linux-mm@...ck.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, aarcange@...hat.com,
        aaron.lu@...el.com, akpm@...ux-foundation.org,
        alex.williamson@...hat.com, bsd@...hat.com,
        darrick.wong@...cle.com, dave.hansen@...ux.intel.com,
        jgg@...lanox.com, jwadams@...gle.com, jiangshanlai@...il.com,
        mhocko@...nel.org, mike.kravetz@...cle.com,
        Pavel.Tatashin@...rosoft.com, prasad.singamsetty@...cle.com,
        rdunlap@...radead.org, steven.sistare@...cle.com,
        tim.c.chen@...el.com, tj@...nel.org, vbabka@...e.cz
Subject: Re: [RFC PATCH v4 00/13] ktask: multithread CPU-intensive kernel work

On 5 Nov 2018, at 21:20, Daniel Jordan wrote:

> Hi Zi,
>
> On Mon, Nov 05, 2018 at 01:49:14PM -0500, Zi Yan wrote:
>> On 5 Nov 2018, at 11:55, Daniel Jordan wrote:
>>
>> Do you think if it makes sense to use ktask for huge page migration (the data
>> copy part)?
>
> It certainly could.
>
>> I did some experiments back in 2016[1], which showed that migrating one 2MB page
>> with 8 threads could achieve 2.8x throughput of the existing single-threaded method.
>> The problem with my parallel page migration patchset at that time was that it
>> has no CPU-utilization awareness, which is solved by your patches now.
>
> Did you run with fewer than 8 threads?  I'd want a bigger speedup than 2.8x for
> 8, and a smaller thread count might improve thread utilization.

Yes. When migrating one 2MB THP with migrate_pages() system call on a two-socket server
with 2 E5-2650 v3 CPUs (10 cores per socket) across two sockets, here are the page migration
throughput numbers:

             throughput       factor
1 thread      2.15 GB/s         1x
2 threads     3.05 GB/s         1.42x
4 threads     4.50 GB/s         2.09x
8 threads     5.98 GB/s         2.78x

>
> It would be nice to multithread at a higher granularity than 2M, too: a range
> of THPs might also perform better than a single page.

Sure. But the kernel currently does not copy multiple pages altogether even if a range
of THPs is migrated. Page copy function is interleaved with page table operations
for every single page.

I also did some study and modified the kernel to improve this, which I called
concurrent page migration in https://lwn.net/Articles/714991/. It further
improves page migration throughput.


—
Best Regards,
Yan Zi

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