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

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Date:	Fri, 25 Apr 2014 20:03:03 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Ming Lei <tom.leiming@...il.com>
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

On 2014-04-25 18:01, Ming Lei wrote:
> Hi Jens,
>
> On Sat, Apr 26, 2014 at 5:23 AM, Jens Axboe <axboe@...nel.dk> wrote:
>> On 04/25/2014 03:10 AM, Ming Lei wrote:
>>
>> Sorry, I did run it the other day. It has little to no effect here, but
>> that's mostly because there's so much other crap going on in there. The
>> most effective way to currently make it work better, is just to ensure
>> the caching pool is of a sane size.
>
> Yes, that is just what the patch is doing, :-)

But it's not enough. For instance, my test case, it's 255 tags and 64 
CPUs. We end up in cross-cpu spinlock nightmare mode.

> From percpu_ida view, it is easy to observe it can improve
> allocation performance. I have several patches to export
> these information by sysfs for monitoring percpu_ida
> performance.

Sounds good!

>> I've got an alternative tagging scheme that I think would be useful for
>> the cases where the tag space to cpu ratio isn't big enough. So I think
>> we'll retain percpu_ida for the cases where it can cache enough, and
>> punt to an alternative scheme when not.
>
> OK, care to comment on the patch or the idea of setting percpu cache
> size as (nr_tags / hctx->nr_ctx)?

I think it's a good idea. The problem is that for percpu_ida to be 
effective, you need a bigger cache than the 3 I'd get above. If that 
isn't the case, it performs poorly. I'm just not convinced the design 
can ever work in the realm of realistic queue depths.


>> That doesn't mean we should not improve percpu_ida. There's quite a bit
>> of low hanging fruit in there.
>
> IMO percpu_max_size in percpu_ida is very important for the
> performance, and it might need to adjust dynamically according
> to the percpu allocation loading, but it is far more complicated
> to implement. And it might be the simplest way to fix the parameter
> before percpu_ida_init().

That's what I did, essentially. Ensuring that the percpu_max_size is at 
least 8 makes it a whole lot better here. But still slower than a 
regular simple bitmap, which makes me sad. A fairly straight forward 
cmpxchg based scheme I tested here is around 20% faster than the bitmap 
approach on a basic desktop machine, and around 35% faster on a 
4-socket. Outside of NVMe, I can't think of cases where that approach 
would not be faster than percpu_ida. That means all of SCSI, basically, 
and the basic block drivers.

-- 
Jens Axboe

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