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

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Message-ID: <20170315141813.GB32626@dhcp22.suse.cz>
Date:   Wed, 15 Mar 2017 15:18:14 +0100
From:   Michal Hocko <mhocko@...nel.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>,
        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 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
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?
-- 
Michal Hocko
SUSE Labs