linux-kernel - Re: [RFC PATCH v2 6/7] lib/persubnode: Introducing a simple per-subnode APIs

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Date:	Tue, 12 Jul 2016 14:51:31 -0400
From:	Waiman Long <waiman.long@....com>
To:	Tejun Heo <tj@...nel.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.com>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	<linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Dave Chinner <dchinner@...hat.com>,
	Boqun Feng <boqun.feng@...il.com>,
	Scott J Norton <scott.norton@....com>,
	Douglas Hatch <doug.hatch@....com>
Subject: Re: [RFC PATCH v2 6/7] lib/persubnode: Introducing a simple per-subnode
 APIs

On 07/12/2016 10:27 AM, Tejun Heo wrote:
> Hello,
>
> On Mon, Jul 11, 2016 at 01:32:11PM -0400, Waiman Long wrote:
>> The percpu APIs are extensively used in the Linux kernel to reduce
>> cacheline contention and improve performance. For some use cases, the
>> percpu APIs may be too fine-grain for distributed resources whereas
>> a per-node based allocation may be too coarse as we can have dozens
>> of CPUs in a NUMA node in some high-end systems.
>>
>> This patch introduces a simple per-subnode APIs where each of the
>> distributed resources will be shared by only a handful of CPUs within
>> a NUMA node. The per-subnode APIs are built on top of the percpu APIs
>> and hence requires the same amount of memory as if the percpu APIs
>> are used. However, it helps to reduce the total number of separate
>> resources that needed to be managed. As a result, it can speed up code
>> that need to iterate all the resources compared with using the percpu
>> APIs. Cacheline contention, however, will increases slightly as each
>> resource is shared by more than one CPU. As long as the number of CPUs
>> in each subnode is small, the performance impact won't be significant.
>>
>> In this patch, at most 2 sibling groups can be put into a subnode. For
>> an x86-64 CPU, at most 4 CPUs will be in a subnode when HT is enabled
>> and 2 when it is not.
> I understand that there's a trade-off between local access and global
> traversing and you're trying to find a sweet spot between the two, but
> this seems pretty arbitrary.  What's the use case?  What are the
> numbers?  Why are global traversals often enough to matter so much?

The last 2 RFC patches were created in response to Andi's comment to 
have coarser granularity than per-cpu. In this particular use case, I 
don't think global list traversals are frequent enough to really have 
any noticeable performance impact. So I don't have any benchmark number 
to support this change. However, it may not be true for other future use 
cases.

These 2 patches were created to gauge if using a per-subnode API for 
this use case is a good idea or not. I am perfectly happy to keep it as 
per-cpu and scrap the last 2 RFC patches. My main goal is to make this 
patchset more acceptable to be moved forward instead of staying in limbo.

Cheers,
Longman