linux-kernel - Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when stealing tags

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Date:	Wed, 23 Apr 2014 08:53:48 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Alexander Gordeev <agordeev@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Kent Overstreet <kmo@...erainc.com>,
	Shaohua Li <shli@...nel.org>,
	Nicholas Bellinger <nab@...ux-iscsi.org>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH RFC 0/2] percpu_ida: Take into account CPU topology when
 stealing tags

Hi Jens,

On Tue, Apr 22, 2014 at 11:57 PM, Jens Axboe <axboe@...nel.dk> wrote:
> On 04/22/2014 08:03 AM, Jens Axboe wrote:
>> On 2014-04-22 01:10, Alexander Gordeev wrote:
>>> On Wed, Mar 26, 2014 at 02:34:22PM +0100, Alexander Gordeev wrote:
>>>> But other systems (more dense?) showed increased cache-hit rate
>>>> up to 20%, i.e. this one:
>>>
>>> Hello Gentlemen,
>>>
>>> Any feedback on this?
>>
>> Sorry for dropping the ball on this. Improvements wrt when to steal, how
>> much, and from whom are sorely needed in percpu_ida. I'll do a bench
>> with this on a system that currently falls apart with it.
>
> Ran some quick numbers with three kernels:
>
> stock           3.15-rc2
> limit           3.15-rc2 + steal limit patch (attached)

I am thinking/working on this sort of improving too, but my
idea is to compute tags->nr_max_cache by below:

                   nr_tags / hctx->max_nr_ctx

hctx->max_nr_ctx means the max sw queues mapped to the
hw queue, which need to be introduced in the approach, actually,
the value should represent the CPU topology info.

It is a bit complicated to compute hctx->max_nr_ctx because
we need to take account into CPU hotplug and probable
user-defined mapping callback.

If user-defined mapping callback needn't to be considered, the
hctx->max_nr_ctx can be figured out before mapping sw
queue in blk_mq_init_queue() by supposing each CPU is
online first, once it is done, the map for offline CPU is cleared,
then start to call blk_mq_map_swqueue().

In my null_blk test on a quad core SMP VM:

     - 4 hw queue
     - timer mode

With the above approach, tag allocation from local CPU can be
improved from:

        5%   -> 50% for boot CPU
        30% -> 90% for non-boot CPU.

If no one objects the idea, I'd like to post a patch for review.

Thanks,
-- 
Ming Lei
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