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

From: "Zi Yan" <zi.yan@cs.rutgers.edu>
To: "Daniel Jordan" <daniel.m.jordan@oracle.com>
Cc: linux-mm@kvack.org, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, aarcange@redhat.com,
	aaron.lu@intel.com, akpm@linux-foundation.org,
	alex.williamson@redhat.com, bsd@redhat.com,
	darrick.wong@oracle.com, dave.hansen@linux.intel.com,
	jgg@mellanox.com, jwadams@google.com, jiangshanlai@gmail.com,
	mhocko@kernel.org, mike.kravetz@oracle.com,
	Pavel.Tatashin@microsoft.com, prasad.singamsetty@oracle.com,
	rdunlap@infradead.org, steven.sistare@oracle.com,
	tim.c.chen@intel.com, tj@kernel.org, vbabka@suse.cz
Subject: Re: [RFC PATCH v4 00/13] ktask: multithread CPU-intensive kernel work
Date: Mon, 05 Nov 2018 21:48:56 -0500	[thread overview]
Message-ID: <7E53DD63-4955-480D-8C0D-EB07E4FF011B@cs.rutgers.edu> (raw)
In-Reply-To: <20181106022024.ndn377ze6xljsxkb@ca-dmjordan1.us.oracle.com>

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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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