linux-kernel - Re: [PATCH v2 0/5] mm: support parallel free of memory

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Date:   Wed, 15 Mar 2017 23:44:07 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ying Huang <ying.huang@...el.com>
Subject: Re: [PATCH v2 0/5] mm: support parallel free of memory

On Wed, Mar 15, 2017 at 03:18:14PM +0100, Michal Hocko wrote:
> On Wed 15-03-17 16:59:59, Aaron Lu wrote:
> [...]
> > The proposed parallel free did this: if the process has many pages to be
> > freed, accumulate them in these struct mmu_gather_batch(es) one after
> > another till 256K pages are accumulated. Then take this singly linked
> > list starting from tlb->local.next off struct mmu_gather *tlb and free
> > them in a worker thread. The main thread can return to continue zap
> > other pages(after freeing pages pointed by tlb->local.pages).
> 
> I didn't have a look at the implementation yet but there are two
> concerns that raise up from this description. Firstly how are we going
> to tune the number of workers. I assume there will be some upper bound
> (one of the patch subject mentions debugfs for tuning) and secondly

The workers are put in a dedicated workqueue which is introduced in
patch 3/5 and the number of workers can be tuned through that workqueue's
sysfs interface: max_active.

> if we offload the page freeing to the worker then the original context
> can consume much more cpu cycles than it was configured via cpu
> controller. How are we going to handle that? Or is this considered
> acceptable?

I'll need to think about and take a look at this subject(not familiar
with cpu controller).

Thanks.