linux-kernel - Re: [PATCH 0/5] mm: support parallel free of memory

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Message-Id: <20170228163947.cbd83e48dcb149c697b316cd@linux-foundation.org>
Date:   Tue, 28 Feb 2017 16:39:47 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Aaron Lu <aaron.lu@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Ying Huang <ying.huang@...el.com>
Subject: Re: [PATCH 0/5] mm: support parallel free of memory

On Fri, 24 Feb 2017 19:40:31 +0800 Aaron Lu <aaron.lu@...el.com> wrote:

> For regular processes, the time taken in its exit() path to free its
> used memory is not a problem. But there are heavy ones that consume
> several Terabytes memory and the time taken to free its memory could
> last more than ten minutes.
> 
> To optimize this use case, a parallel free method is proposed here.
> For detailed explanation, please refer to patch 2/5.
> 
> I'm not sure if we need patch 4/5 which can avoid page accumulation
> being interrupted in some case(patch description has more information).
> My test case, which only deal with anon memory doesn't get any help out
> of this of course. It can be safely dropped if it is deemed not useful.
> 
> A test program that did a single malloc() of 320G memory is used to see
> how useful the proposed parallel free solution is, the time calculated
> is for the free() call. Test machine is a Haswell EX which has
> 4nodes/72cores/144threads with 512G memory. All tests are done with THP
> disabled.
> 
> kernel                             time
> v4.10                              10.8s  __2.8%
> this patch(with default setting)   5.795s __5.8%

Dumb question: why not do this in userspace, presumably as part of the
malloc() library?  malloc knows where all the memory is and should be
able to kick off N threads to run around munmapping everything?