linux-kernel - Re: [patch 1/2]percpu

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Date:	Sun, 9 Feb 2014 16:50:07 +0100
From:	Alexander Gordeev <agordeev@...hat.com>
To:	Kent Overstreet <kmo@...erainc.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Shaohua Li <shli@...nel.org>,
	linux-kernel@...r.kernel.org, hch@...radead.org
Subject: Re: [patch 1/2]percpu_ida: fix a live lock

On Mon, Jan 06, 2014 at 01:47:26PM -0800, Kent Overstreet wrote:
> Ok, so I hadn't really given any thought to that kind of use case; insofar as I
> had I would've been skeptical percpu tag allocation made sense for 32 different
> tags at all.
> 
> We really don't want to screw over the users that aren't so constrained by the
> size of their tag space; there really is a huge performance tradeoff here
> (otherwise you're stealing tags and bouncing cachelines for _every_ tag
> allocation when the queue is full, and your percpu tag allocation is no longer
> very percpu).
> 
> I'm not sure what the best strategy is for NCQ-type max nr_tags, though -
> thoughts?
> 
> Easy thing to do for now is just to add another parameter to percpu_ida_init()
> for the number of tags that are allowed to sit unused on other cpu's freelists -
> users that have large relatively unbounded nr_tags can set that to nr_tags / 2,
> for NCQ you'd set it to 0.
> 
> I suspect we can do better for NCQ though, w.r.t. worst case performance.

Yeah, that was my first thought when I posted "percpu_ida: Allow variable
maximum number of cached tags" patch some few months ago. But I am back-
pedalling as it does not appear solves the fundamental problem - what is the
best threshold?

May be we can walk off with a per-cpu timeout that flushes batch nr of tags
from local caches to the pool? Each local allocation would restart the timer,
but once allocation requests stopped coming on a CPU the tags would not gather
dust in local caches.

-- 
Regards,
Alexander Gordeev
agordeev@...hat.com
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