linux-kernel - Re: [PATCH] mm/vmalloc: Introduce DEBUG_VMALLOCINFO to reduce spinlock contention

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Date:	Thu, 17 Apr 2014 16:18:28 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Richard Yao <ryao@...too.org>
Cc:	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Wanpeng Li <liwanp@...ux.vnet.ibm.com>,
	Johannes Weiner <hannes@...xchg.org>,
	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	kernel@...too.org, Matthew Thode <mthode@...ode.org>
Subject: Re: [PATCH] mm/vmalloc: Introduce DEBUG_VMALLOCINFO to reduce
 spinlock contention

On Thu, 10 Apr 2014 12:40:58 -0400 Richard Yao <ryao@...too.org> wrote:

> Performance analysis of software compilation by Gentoo portage on an
> Intel E5-2620 with 64GB of RAM revealed that a sizeable amount of time,
> anywhere from 5% to 15%, was spent in get_vmalloc_info(), with at least
> 40% of that time spent in the _raw_spin_lock() invoked by it.

This means that something in userspace is beating the crap out of
/proc/meminfo.  What is it and why is it doing this?

/proc/meminfo reads a large amount of stuff and gathering it will
always be expensive.  I don't think we really want to be doing
significant work and adding significant complexity to optimize meminfo.

If there really is a legitimate need to be reading meminfo with this
frequency then it would be pretty simple to optimise
get_vmalloc_info(): all it does is to return two ulongs and we could
maintain those at vmalloc/vfree time rather than doing the big list
walk.

If we can address these things then the vmap_area_lock problem should
just go away - the kernel shouldn't be calling vmalloc/vfree at high
frequency, especially during a compilation workload.
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