Date: Thu, 17 Apr 2014 16:18:28 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Richard Yao <ryao@gentoo.org>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Zhang Yanfei <zhangyanfei.yes@gmail.com>,
        Wanpeng Li <liwanp@linux.vnet.ibm.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        HATAYAMA Daisuke <d.hatayama@jp.fujitsu.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, kernel@gentoo.org,
        Matthew Thode <mthode@mthode.org>
Subject: Re: [PATCH] mm/vmalloc: Introduce DEBUG_VMALLOCINFO to reduce
 spinlock contention
Message-Id: <20140417161828.b1740b30bf9d5462f46562cc@linux-foundation.org>
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On Thu, 10 Apr 2014 12:40:58 -0400 Richard Yao <ryao@gentoo.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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