linux-kernel - Re: Solving M produces N consumers scalability problem

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Date:	Mon, 4 Nov 2013 12:23:06 -0800
From:	Anatol Pomozov <anatol.pomozov@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Solving M produces N consumers scalability problem

Hi

Thanks for your reply

On Mon, Nov 4, 2013 at 10:53 AM, Andi Kleen <andi@...stfloor.org> wrote:
> Anatol Pomozov <anatol.pomozov@...il.com> writes:
>>
>> One idea is not to use the spin_lock. It is the 'fair spin_lock' that
>> has scalability problems
>> http://pdos.csail.mit.edu/papers/linux:lock.pdf Maybe lockless
>> datastructures can help here?
>
> The standard spin lock is already improved.
> But better locks just give you a small advantage, they don't
> solve the real scaling problem.

Do you know in what version the improvement happened? I use kernel 3.3
and I can backport the changes to my custom kernel to make the
situation better.

>> Another idea is avoid global datasctructures but I have a few
>> questions here. Let's say we want to use per-CPU lists. But the
>> problem is that producers/consumers are not distributed across all
>> CPUs. Some CPU might have too many producers, some other might not
>> have consumers at all. So we need some kind of migration from hot CPU
>> to the cold one. What is the best way to achieve it? Are there any
>> examples how to do this? Any other ideas?
>
> per cpu is the standard approach, but usually overkill. Also
> requires complex code to drain etc.
>
> Some older patches also use per node, but that works very poorly
> these days (nodes are far too big)
>
> One way I like is to simply use a global (allocated) array of queues,
> sized by total number of possible cpus (but significantly smaller) and
> use the cpu number as a hash into the array.

This solution pretty-much equivalent to per-CPU data structures. And
in this case there also will be "hot" nodes and "cold" nodes. So my
question remains - what is the best way to implement data migration
between nodes, is there a standard solution for this? examples?
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